Accurate lattice parameters a and c of the tetragonal chalcopyrite quaternary semiconductor CuGaSnl3Se4 have been determined as a function of temperature by the X-ray powder diffraction method in the temperature range 300 K to about 900 K. The data have been used to evaluate the axial expansion coefficients aa and a c at various temperatures. The thermal expansion studies revealed the anisotropy between the axial expansion coefficients having a larger coefficient of expansion along the a-axis than that along the c-axis (aa > ac). The mean values a, and ac, in the temperature range 300-900 K, are found to be 14.02 x 10-6K 1 and 5.02 x 10-6K 1 respectively, and the axial ratio, c/a, changes with a coefficient of -8.96 x 10 -6 K -1. This result indicates an increase in the tetragonal distortion, 6 = 2 -c/a with temperature. An attempt is made to explain the increase in tetragonal distortion with temperature and the anisotropic thermal expansion of CuGaSnE]Se4 in terms of the thermal expansion of the A-Se (where A is Cu and Ga randomly distributed) and B-Se (where B is Sn and vacancy randomly distributed) bonds. The results are also discussed in terms of the principal Gr/ineisen parameters of chalcopyrite structure compounds.