COLUMBIA, S.C.  A legislative committee will convene next week to begin preliminary impeachment proceedings against Gov. Mark Sanford, according to a letter sent Friday by the chairman of the state’s House Judiciary Committee.

The letter, from State Representative James H. Harrison, a Republican, said an ad hoc committee would review a resolution to impeach Mr. Sanford on charges of “serious misconduct in office” and dereliction of duty after he secretly left the state in June, leaving no one in charge, to visit a woman in Argentina with whom he was having an extramarital affair.

State lawmakers have considered impeachment for months, but this announcement is the first formal consideration of such a move. It came two days after the State Ethics Commission said it had found probable cause to hear ethics complaints against Mr. Sanford, also a Republican.

The commission, whose nine members were appointed by the governor, did not say what charges it would pursue, but it reviewed several accusations against Mr. Sanford: that he had used state airplanes for personal or political reasons, bought high-priced plane tickets in violation of policy, failed to disclose trips on private planes and misspent campaign contributions.