Abstract The crystal structures of GdCuAs 2, GdCuAs 1.15P 0.85, and GdCuP 2.20 have been investigated by the single crystal and powder methods. While GdCuAs 2 ( P4/ nmm) retains the tetragonal HfCuSi 2 structure (M. Brylak, M. H. Möller, and W. Jeitschko, J. Solid State Chem. 115, 305 (1995)), GdCuAs 1.15P 0.85 ( Pmmn, a=3.849(1), b=3.8501(1), c=9.871(3) Å) and GdCuP 2.20 ( Pmm2, a=5.3747(9), b=5.3830(9), c=9.7376(16) Å) undergo orthorhombic distortions. The changes are significant in GdCuP 2.20: P dimers are formed in the P layer along the a direction and there is an additional (but deficient) P site on one side of the layer that links the dimers. The GdCuAs 1.15P 0.85 structure was predicted by the Landau theory. According to this theory the transition from GdCuAs 2 to GdCuAs 1.15P 0.85 can be a continous one, the transition from GdCuAs 1.15P 0.85 to GdCuP 2.20 is a first-order one. The transition to GdCuP 2.20 occurs with twin formation. The electronic structure and bonding are analyzed by the extended Hückel tight-binding method. The conductivity and magnetic measurements for the arsenide and phosphide are reported.