The quantitative evaluation of the constituents of mixtures from x‐ray powder diffraction patterns has been highly successful in certain cases, but has been found subjected to limitations as an analytical tool of more general use. These arise from a number of effects which interfere with a simple interpretation of the patterns. The paper discusses the basic aspects of these interferences, in particular of those which result from superpositions of lines, and develops a method which takes account of them. The method is adapted to meet such cases where the constituents or impurities which interfere with the regular patterns are unknown.