WASHINGTON, Aug. 6 — H. Stewart Van Scoyoc, founder of one of the biggest lobbying firms in Washington, spent an anxious morning with his lawyer last week assessing the far-reaching ethics and lobbying rules Congress had passed the day before.

The first worry was what lobbyists are calling the new “temptation rules.” Not only do they bar lawmakers and aides from accepting any gifts, meals or trips from lobbyists, they also impose penalties up to $200,000 and five years in prison on any lobbyist who provides such freebies.

And worse still for Mr. Van Scoyoc, under the new law he is required to certify each quarter that none of the 50 lobbyists in his firm bought so much as a burger or cigar for someone on a lawmaker’s staff.

“You are basically asking people to certify, with big penalties, that nobody has lied on their expense accounts,” Mr. Van Scoyoc said, marveling at the complexity of policing such casual contact between lobbyists and Congressional aides. “These are people who are sharing apartments together, playing on the same softball teams, dating each other—young people with active social lives.”