The determination of the (Fe, Mg)-cation distribution in important rock forming minerals such as olivines and orthopyroxenes from single crystal X-ray diffraction data is well established. The findings obtained from good quality data are, however, subject to the structure model, refinement strategy, data weighting etc. Moreover, they do not necessarily agree with those of comparable Mössbauer studies. In order to assess the reliability of the X-ray results, i.e. precision and accuracy of the site occupancies and ensuing cation distribution coefficients K D , an interlaboratory study was undertaken. Two olivine and two orthopyroxene data sets, one experimental and three synthetic, were dispatched to interested applicants for structure refinements according to their own choice. Approximately correct structure parameters were given, but no information about the chemical composition, i.e. the Fe-content, was communicated. The total of 25 refinements for each data set, carried out with different programs, valence models, and weighting schemes, yielded some considerable differences between the results, which are summarized and discussed in terms of precision and accuracy. The uncertainties of the refined site occupancies, following from the qualities of the fits, are only realistic, when they are dominated by the statistical errors in the data. Errors in the independently refined Fe-concentrations on the M1-and M2-sites are not fully transferred to the thermodynamically relevant cation distribution coefficient. Therefore, correlations between the site occupancies, clearly recognizable from the compilations of all respective results, can be combined with an independently obtained chemical constraint (microprobe analysis) in order to yield reliable K D -values with realistic uncertainties. In summary, all results indicate the need for multiple refinement techniques, if reliable results on the absolute scale are desired.