Ultraviolet photon induced bulk, surface and interface modifications in n-Hg0.8Cd0.2Te (MCT) and metal insulator semiconductor (MIS) structures fabricated from CdTe/n-Hg0.8Cd0.2Te have been investigated in a hydrogen environment. The shift of the absorption edge towards the short wavelength side and the enhancement of the IR transmission in n-Hg0.8Cd0.2Te are thought to be due to the reduction in the density of gap states due to residual impurities or defects. A pre-CdTe deposition surface treatment prevents the surface inversion of n-MCT during CdTe deposition and MIS devices after UV photon excitation show a considerable lowering of the interface charges. X-ray photoelectron spectroscopy profiles of annealed CdTe/n-Hg0.8Cd0.2Te show evidence of interface grading which accounts for the stability of CdTe passivation.