RCu–As–S thin films were deposited using the thermal evaporation technique. Optical transmission spectra of Cu0.1As2.1S3.1 thin films were measured within the temperature range 77…300 K. Temperature behaviour of absorption edge inherent to Cu0.1As2.1S3.1 thin films was studied. Temperature and compositional dependences of optical parameters in Cu–As–S thin films have been analyzed. It has been revealed that the energy pseudogap decrease and Urbach energy increase with the copper content increase take place in Cu–As–S thin films. The influence of order-disorder processes on optical properties of Cu–As–S thin films has been discussed.