The cation distributions in copper ferrite-chromite series and the copper ferrite quenched from high temperatures are measured by X-ray diffraction method. The lattice parameters are also measured as a function of chromium ion content and of temperature. The critical fractions of copper ions on 16d and 8a sites for the occurrence of bulk crystal distortion from cubic to tetragonal symmetry are determined. The critical temperature for the distortion of copper ferrite is 360°C. This crystal distortion depends only upon the copper ion distribution. The abrupt appearance of distortion and coexistence of two phases, cubic and tetragonal, near the critical fraction and the critical temperature suggest that this distortion is of the first order as predicted by the statistical theory. From the aging of the distortion, the activation energy for copper ion to migrate from 16d site to 8a site is obtained. From the comparison of the critical fraction of copper ion in this series with those in other mixed spinels, the origin of this crystal distortion is discussed.