Those relationships will be under scrutiny on Tuesday by a Senate panel, which plans to examine possible conflicts of interest that arise when brokerage firms are paid to direct their customers’ orders to particular trading venues.

Behind the sleek trading interfaces of brokerage firms like TD Ameritrade, Charles Schwab, and Merrill Lynch lie a web of business relationships with relatively obscure firms that make trades happen.

To the average investor with a brokerage account, the process of buying and selling shares of stock seems straightforward. But the back end of these systems, governing how billions of shares are traded, remains opaque to many customers.


The Senate panel, the Permanent Subcommittee on Investigations, has summoned for questioning stock exchange executives, as well as officials from a mutual fund company and a brokerage firm.

The heightened attention from lawmakers comes as regulators are also taking a close look at the stock market, which has been transformed during the past decade into a place dominated by high-speed traders and fragmented across multiple exchanges.

The head of the Securities and Exchange Commission, Mary Jo White, recommended earlier this month a set of new rules intended to update market regulation for the electronic era.

But while White’s proposals were broad in their scope, touching high-frequency trading and the strength of the market’s structure, the Senate panel on Tuesday plans to zero in on the question of fairness.

The panel, led by Senator Carl Levin, a Michigan Democrat, will examine a payment system that has become endemic on Wall Street, though it is little known outside the world of high finance.

Brokerage firms have a regulatory obligation to give their customers the best possible terms when executing their stock orders. The execution process frequently involves routing the orders to an exchange or a trading firm that can find a match and consummate the order.


But the brokerage firms do not give away their customers’ orders. In many cases, they accept fees or rebates from trading firms or exchanges that receive the orders, under arrangements called “payment for order flow” and the “maker-taker” pricing model.

The question before the Senate panel is whether these payments compromise the brokerage firms’ duty to serve their customers.

Brokers who route orders to the venues with the highest fees “may not be obtaining best execution for their clients,” Robert H. Battalio, a professor of finance at the University of Notre Dame, will say on Tuesday, according to a copy of his prepared remarks.

Citing academic research, he plans to say: “Our results suggest that the decision to use a single venue that offers the highest liquidity rebates is not consistent with the objective of obtaining best execution for customer limit orders.”

The hearing may produce fireworks. On the lineup of witnesses is Bradley Katsuyama, the chief executive of an upstart trading venue called IEX Group, who rose to prominence after being featured in “Flash Boys,” a recent book by Michael Lewis that portrays high-frequency traders as market villains.

Katsuyama has strongly criticized aspects of high-frequency trading, which deploys computer algorithms to buy and sell shares in milliseconds.

IEX is trying to gain legitimacy as a full-fledged exchange. The company said Monday that it had hired John Ramsay, the former acting head of the trading and markets division of the SEC, to oversee regulatory compliance.


But while Katsuyama has built a business on the idea that high-speed trading can be harmful to investors, another witness on Tuesday, Joseph P. Ratterman, the chief executive of an exchange company called BATS Global Markets, will probably defend aspects of the market, including the fees paid to brokerage firms.

IEX and BATS have something of a history. Katsuyama appeared on CNBC in April, shortly after “Flash Boys” was released, to spar with another BATS executive, William O’Brien.

Insults flew.

“I believe the markets are rigged. I also think you’re part of the rigging,” Katsuyama told O’Brien, after the exchange executive accused his interlocutor of “possibly scaring millions of investors in an effort to promote a business model.”

Other witnesses may deliver measured calls for reform. The New York Stock Exchange, which adopted the order fees after years of resisting them, will be represented on the witness stand by Thomas W. Farley, its president.

Jeffrey C. Sprecher, the chief executive of IntercontinentalExchange, the company that recently bought the NYSE, has recently ramped up his criticism of “payment for order flow” and “maker-taker” structures.

But rather than act unilaterally to end the Big Board’s involvement, Sprecher, who will not be testifying on Tuesday, has called on regulators to enact a more comprehensive change.

The lone representative of a brokerage firm on Tuesday will be Steven Quirk, a senior vice president at TD Ameritrade. In a timely move, TD Ameritrade said last week that it would start disclosing more information about the payments it receives for routing orders to particular venues.


TD Ameritrade has sought “to ensure that we remain focused on satisfying our obligation to seek best execution on behalf of our clients,” Fredric Tomczyk, the president and chief executive, said in a statement last week. “Without their trust and satisfaction we wouldn’t have a business.”