A little more than two years after the Internal Revenue Service issued bare-bones guidance on bitcoin and other digital currencies, the agency still hasn’t addressed many important tax matters affecting them.

The American Institute of CPAs sent the IRS a letter earlier this month requesting clarifications on 10 issues, including the tax status of small transactions and rules for donating digital currencies to charity.

“We’d like to know the tax rules before they turn into audit issues,” says Annette Nellen, a professor at San Jose State University in Silicon Valley, who helped draft the AICPA’s request.

The tax questions are one of several hurdles on the path to broader acceptance of digital currencies—money that exists only online and isn’t backed by any government. Bitcoin and its smaller rivals, such as ether, ripple and litecoin, are maintained by a network of computers that process and verify transactions using them.

Digital currencies gained favor in 2013 and 2014 when federal and state regulators issued rules for them and businesses such as Overstock.com and the Sacramento Kings basketball team said they would accept bitcoin payments. But volatility and scandals, such as the 2014 failure of Mt. Gox, an early bitcoin exchange, put off some investors.