The development of an optical constant library for Hg1-xCdxTe as a function of composition (x = 0-0.5) and temperature (T=0-250 degrees C) which is suitable for precise composition control by spectroscopic ellipsometry (SE) during MBE growth is described. An efficient methodology for acquiring in situ optical constants as a function of composition and temperature is first presented. Optical constants extracted from these in situ measurements, as well as literature data from room temperature values, were used to obtain internally Kramers-Kronig consistent parametric optical constant models at discrete compositions and temperatures. Then a global data analysis over temperature 'T' and composition 'x' was performed in which the internal parameters of the optical constant model were fitted as polynomials in T and x. This parametric model was developed to replace, without compromising the quality of ellipsometric data fits, the usual tabulated optical constant lists while using a reasonably small set of adjustable parameters. The model is flexible enough to describe the complicated critical point structures of semiconductors, yet stable enough to generate optical constants as a function of composition and temperature and permit limited extrapolation outside the measured range.