A study has been made of the factors which determine the transmission limits of the II–IV–V2 and I–III–VI2 ternary semiconductor compounds in view of their possible use in infra‐red non‐linear devices. The results of new measurements are presented for the following compounds: ZnSiP2, CdSiP2, CuAlS2, CuAlSe2, CuGaS2, CuGaSe2. AgGaS2, AgGaSe2, and AgInS2. Details are given of their fundamental absorption edges, two‐phonon summation bands, and Reststrahl bands. Some conclusions are drawn regarding the mechanisms of band‐edge absorption. A transmission limit is defined as the wavelength at which the absorption coefficient has increased to 3 cm−1. A full summary is included of all known data on the transmission ranges of the principal ternary compounds.