[en] The resolution function of an X-ray triple-crystal diffractometer on a rotating-anode source has been studied both experimentally and theoretically. Three different experimental configurations were used, with perfect germanium, distorted germanium and pyrolytic graphite as monochromator and analyser, leading to changes in the resolution of two orders of magnitude. The results are compared with a Gaussian description of the resolution function and it is found that this describes the width of the central part with an accuracy of better than 25%. The wings of the resolution in the high-resolution mode show the effect of the non-Gaussian resolution elements but these are reproduced by a full numerical convolution theory of the resolution function. (orig.)