A novel experimental technique was used to investigate the in-cylinder air-fuel ratio excursions of a port-injected spark-ignition engine during load and fuel transients. This involved sampling directly from the engine cylinder using a fast flame ionisation detector (FID) system throughout an engine transient test. All tests were conducted with the coolant at the normal operating temperature of 90°C.The research engine used was a 1.6ltr four-cylinder multi-point fuel injection spark-ignition (SI) engine with four-valves-per-cylinder, with sequential injection and an electronic management system. The engine transient involved a rapid throttle opening within about 15msec. Various load steps were investigated at 2000rev/min along with the effect of altering the type of fuel injector. It was found that the response of the engine to a load transient involved a lean air-fuel ratio excursion (maximum gravimetric air-fuel ratio of about 25:1) lasting for only a single cycle, followed by a rich air-fuel ratio excursion (minimum air-fuel ratio of about 10:1) which decayed towards stoichiometric air-fuel ratio after about 10 cycles.