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After discussing some of the basic ideas, advantages and disadvantages of X‐ray powder diffraction analysis, it is pointed out that at present a major disadvantage for colour chemists is the lack of a special sub‐file of suitable standard X‐ray data. A plea is made for colleagues to report their data, forthe Journal to act as a depository and for the Society to consider the feasibility of separate data sub‐file of use to colour chemists and dyers, persuading the Joint Committee on Powder Diffraction Standards (JCPDSj to set one up. The article then proceeds to illustrate the usefulness of the technique to problems in colour chemistry. The effects on the X‐ray powder pattern of the crystal size, polymorphism, isomers, and solid solutions and mixed crystals are discussed. This is illustrated with examples from the work of the author and other investigators. The paper concludes by looking at the application of the X‐ray powder method to the analysis of vat dyes on various fabrics.