An experimental approach based on the simultaneous measurement of laser-induced fluorescence along a line and the transmission is discussed which allows the quantitative measurement of combustion intermediates. The technique is applied to measure in-cylinder number densities of OH radicals using an optically accessible two-stroke engine. OH radicals are recorded for different cylinder pressures during the expansion stroke. From the data pressure dependent probability density functions (pdfs) are obtained. Precalculated combustion scenarios based on an opposed jet geometry and on a homogeneous reaction scheme are used to analyse correlations between the measured OH number densities and other scalars, e.g, the temperature and O-atoms which are important for the detennination of NOxemissions in post-processing steps of CFD calculations. By the use of experimentally obtained pdfs for the OH radical and the correlation analyses, an improved estimation of pdfs for the temperature and O-atoms is presented.