Crystalline samples of the alloy Cd(Ga1−xInx)2Se4 are synthesized and the lattice parameters are determined by X‐ray diffraction. A complete solubility is obtained in the whole composition range, but for intermediate compounds a discontinuity in the lattice parameters is observed. This fact should indicate that a phase transition from the I4 structure of CdGa2Se4 to the I42m structure of CdIn2Se4 takes place through a disordered intermediate phase.