An experimental study was made to determine the time history of fuel vapor concentration in the combustion chamber of an automotive spark ignition engine. The laser Rayleigh scattering was applied for the remote, nonintrusive, point-probing of the concentration of fuel vapor in the combustion chamber, which was caused by the timed injection of liquid fuel into an intake port. In order to make the optical diagnostics accessible, the original engine was modified to permit installation of an extended combustion cylinder with three sets of glass windows and an extended piston. The results showed that the vapor concentration increased and reached a peak after which it decreased during the intake stroke, and was almost constant during the compression stroke. It was also found that the vapor concentration increased with a reduced air fuel ratio supplied, and with increased engine speed and fuel injection pressure. A maximum occurred as a function of fuel injection timing.