Abstract The (CuGaSe 2 ) 1− x (MgSe) x alloy system (0 < x ≤ 0.5) was investigated using X‐ray powder diffraction, energy dispersion spectrometry and differential thermal analysis. The solubility of MgSe in CuGaSe 2 was found to be nearly complete for all the compositions studied, although traces of MgSe appear as a secondary phase at x ≥ 0.15. All the alloys showed the chalcopyrite structure and the lattice parameters of the unit cell do not follow a linear behavior but showed a soft local maximum at x ∼ 0.15. In the single‐phase field, the increasing behavior of the lattice parameters can be reproduced using an extension for quaternary alloys of Jaffe and Zunger's model for chalcopyrites. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)