The application of deuteriation of both ligands and solvents on the photophysical properties of transition metal complexes in solution and glassy matrices is reviewed. The reduction in amplitude and frequency of vibrational modes due to deuterium's increased mass, relative to hydrogen, has a significant effect on non-radiative deactivation processes, which can occur through both intra- and inter-molecular vibrational coupling. The effect of deuteriation on excited state lifetimes has allowed for its application in probing the nature of excited state decay processes. The effects of isotopic exchange on vibrational spectroscopies such as resonance Raman and low temperature high-resolution emission spectroscopies are also addressed briefly.