Spectral ellipsometry has been applied to in situ diagnostics of Hg1-xCdxTe molecular beam epitaxial growth. Spectral ellipsometry provides surface, composition, and film thickness properties of thin film electronic materials in real-time during growth and processing. This study is discussed in two parts. In the first pact, the surface state properties of Hg1-xCdxTe and the substrate material Cd1-xZnxTe were studied under ultrahigh vacuum (UHV) conditions. By measuring the spectral ellipsometric quantities Psi and Delta versus temperature under UHV conditions, the presence and desorption temperatures of surface species have been determined. Tellurium stabilized Cd1-xZnxTe surfaces were not observed to exhibit any apparent surface changes during heating under these conditions. Oxidized Cd1-xZnxTe surfaces, however, exhibit discontinuities in Psi and Delta indicative of desorption of a surface species in the 300 degrees C range. Tellurium stabilized Hg1-xCdxTe surfaces also exhibit discontinuities in Psi and Delta occurring at about 177 degrees C. In the second part of the study, dielectric functions at the growth temperature, 180 degrees C, were determined for Hg1-xCdxTe alloys and Cd(0.96)Zn(0.04)4Te. Film compositions were monitored in situ and in real-time during Hg1-xCdxTe epitaxial growth based on this knowledge of growth temperature dielectric functions. An experimental analysis of measurement factors illustrates that compositional sensitivity for in situ Hg1-xCdxTe measurement is limited by uncertainties in the angle and plane of incidence. Simulations show an ultimate multiwavelength sensitivity of significantly better than +/-0.0008 is possible for in situ composition measurements.