An experiment has been carried out to investigate the combustion process in an operating S.I. engine (MPI and Multi- valve), using in-cylinder high speed gas sampling techniques.Measurements have been made of local air fuel ratios and time-resolved concentrations of combustion gases such as CO, CO2, THC, NOx, and O2, particularly focused on spark plug location. The effects of fuel injection timing, swirl generating air motion, sampling location, spark timing, speed and load have been considered.With the end of fuel injection at 120° ATDC on the intake stroke, A/F ratio at spark plug location has the leanest value for standard inlet port, while it has the richest value for swirl generating inlet port. The initial NOx concentration in the unburned gas region, diluted by the residual gases, has been substantially reduced between 5° BTDC and 15° ATDC crank angle prior to combustion. It has also been found that in- cylinder high speed gas sampling method is a useful tool to investigate the combustion phenomena in an operating S.I. engine.