The methods available for the calculation of ratios of partition functions for isotopic molecules were examined critically and compared. A number of calculations were made for carbon as well as for selenium systems using the exact method rather than an approximate one. Improvements were made to the sample handling system of the mass spectrometer to prevent sample cross-contamination and to reduce the time needed for sample changeover. Four sets of experiments were performed. The kinetic isotope effects in the reductions to Se° of Se^V1 and Se^1V were studied as were the isotope exchange reactions between Se° and Se^1V and between Se^1V and Se^V1. Estimates were made of the ratios of rate constants and isotope exchange equilibrium constants for 82Se and 76Se in these systems and these were compared with the theoretical values. / Thesis / Doctor of Philosophy (PhD)