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We report quantitative experimental investigations on the air/fuel distribution in the combustion chamber of a spark ignition engine prior to ignition and during the first stages of combustion. A four cylinder VW four-stroke engine was modified to give optical access to the combustion chamber via the piston. The fuel concentration was visualised by planar laser-induced fluorescence (PLIF). The choice of an appropriate fuel dopant is very important. Several properties have to be considered simultaneously. The most crucial influence results from the sensitivity to quenching by oxygen. Since fuel distributions recorded at different engine operating conditions wore to be compared on a quantitative scale, this effect had to be taken into account most carefully. The long fluorescence lifetime and the extraordinarily low quenching rate of vapour-phase fluoranthene in a high pressure environment as pertaining to engines led to its choice as dopant.