[en] Differences in the experimental results of thin target calibration, using different types of reference standards, are discussed and compared to published data of X-ray production cross sections. To correct for matrix effects in PIXE analysis of thick targets, a computer code is being developed considering the decrease of ionization cross section due to the slowing down of protons, the X-ray attenuation by the target material, and the enhancement of X-ray intensities by secondary X-ray fluorescence. The approximation involved are discussed, and the code is checked by comparison with PIXE analyses of thick samples of known composition. (orig.)