Differential thermal analysis and x-ray methods have been used to characterize the InSb-InSe pseudobinary phase diagram. A monotectic exists at 591°C and 91 mole % InSe, and the composition of the InSb rich melt at the monotectic temperature is 13 mole % InSe. A eutectic occurs at 519°C and 3 mole % InSe and two other phase transitions at 492°C and 460°C were present in the system. The solubility of InSe in InSb is less than 1% and less than 1% of InSb is soluble in InSe. © 1966, The Electrochemical Society, Inc. All rights reserved.