The detn. of C, H and O in org. liq. matrixes such as solvents and different base oils by principal component regression in EDXRS is demonstrated. This method is based on a significant contribution of the light elements to the backscattering of x-rays in the sample. As excitation source a Y secondary target in a Cartesian geometry is used. The prediction error of the PCR calibration with respect to unknown samples is in the region of 4.0 mg g-1 for H, 30.0 mg g-1 for C and 26.0 mg g-1 for O.