Results of the growth of Ga 1-x In x Sb crystals as the pseudobinary Ga 1-x Sb--In x Sb are presented. Special focus is given to the relationship between crystal composition and the cut-off wavelength of transmission for the crystal using Fourier Transform Infrared (FTIR) and Ultra Violet-Visual Wavelength (UV-Vis) spectroscopy. This provides a fast, easy method to determine the composition. A Matlab model to determine the composition of a crystal grown using the Bridgman method is developed. This compares the ideal to the regular solution models for Ga 1-x In x Sb. These two models are used to predict the composition of the grown crystals. The regular solution yields a more accurate pseudo-binary phase diagram and thus a much closer fit to the experimental data.