Fast throttle opening in port-injected gasoline engines often results in a lean air-fuel ratio excursion lasting several engine cycles. Even when the engine is equipped with a three-way catalyst this lean excursion can lead to high tailpipe emissions. This paper will describe an in-cylinder method of measuring these air-fuel ratio excursions, using a fast flame ionisation detector. Examples will be given of air-fuel ratio excursions obtained on a four-valve-per-cylinder sequentially-injected gasoline engine equipped with a lambda sensor. The air-fuel ratio excursions together with measurements of the engine air flow are used to estimate me build up of the fuel film on the inlet manifold walls.Whilst air-fuel ratio excursions have been recorded previously by other investigators, their results were obtained from exhaust gas analysis using fast oxygen sensors. This paper will demonstrate that the duration of the excursions recorded from exhaust gas analysis is overestimated, probably due to mixing of the exhaust gases from successive engine cycles.