This paper reports work on pentenary alloy systems similar to the quarternary system in In1-xGaxP1-yAsy. Such AI1-yBIyCIIIDVI2xEV I2(1-x) and AII1-yBIIyCIVDV2xE V2(1-x) pentenary alloys are mixtures of ternary chalcopyrites of the I‐III‐VI2 and II‐IV‐V2 variety. These materials are of interest because their use can improve the performance of heterojunction electro‐optic devices such as solar cells. This improvement arises because the pentenaries can permit different semiconductor layers to be deposited upon each other such that they are lattice and crystallographically matched, but at the same time have independently adjustable band gaps. More specifically, we have focused on the Cu1-yAgyInS2(1-x)Se2x system which appears to have the best potential of all the possible systems for solar energy applications. Samples were prepared by reacting stoichiometric powder mixtures at about 900 °C. X‐ray diffractometry indicated complete solid solubility over the whole compositional range. Cathodoluminescence was used to determine band gaps at 300 and 77 °K and the spectra indicate that all alloys are direct‐band‐gap semiconductors. Isolattice constant and band‐gap contour maps for the system have been obtained.