Valid inspection techniques are required so that previous incidents of misfueling may be determined on vehicles involved in in-use emission surveillance programs. This study indicates that even brief episodes of misfueling can have a permanent adverse effect on the exhaust emissions of catalyst equipped vehicles. Three (3) 1984 closed-loop, three-way catalyst equipped passenger cars and three (3) 1984 open-loop oxidizing catalyst equipped light duty trucks were selected to monitor emission levels and emission control components during misfueling experiments. Lead detection techniques evaluated were gamma ray analysis for the catalytic converter; Plumbtesmo* paper inspections for the tailpipe, EGR valve, and spark plug, and fuel analysis. All exhaust emission constituents (HC, CO and NOx) increased with misfueling but exhibited varying degrees of recovery with resumed usage of unleaded fuel. Lead deposits were also gradually reduced from emission components with resumption of unleaded fuel usage. Gamma ray analysis of the catalytic converter and Plumbtesmo paper inspections on the EGR valve pintle and the spark plug were the most reliable indicators of previous misfueling with leaded gasoline.