Abstract Polycrystalline samples of quaternary (Cu–III–Se 2 ) 1– x (NbSe) x alloys (III: Ga, In) with x = 1/2 (nominally CuNbGaSe 3 and CuNbInSe 3 ) were prepared by the usual melt and anneal technique. The analysis showed a large solubility of Nb in the ternary structures. The unit cell parameters of CuNbGaSe 3 ( a = 5.617 ± 0.001 Å; c = 11.024 ± 0.003 Å; c / a = 2.02; V = 348 ± 1 Å 3 ) and CuNbInSe 3 ( a = 5.779 ± 0.001 Å; c = 11.620 ± 0.003 Å; c / a = 2.01; V = 388 ± 1 Å 3 ) were very close to those of the respective ternaries, CuGaSe 2 and CuInSe 2 . Nevertheless, indexation of the diffraction pattern of CuNbGaSe 3 suggested, for this alloy, a space group transition from I $ \bar 4 $ 2d to P $ \bar 4 $ 2c analogous to CuFeInSe 3 , recently reported. From DTA measurements it was found that CuNbGaSe 3 and CuNbInSe 3 are ordered samples with order–disorder transition temperatures of ∼976 K and ∼910 K, and melted incongruently at ∼1185 K and ∼1046 K, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)