Phosphorescence measurements with 5,8-dimethyl-1-tetralone (DMT) and 5,8-dimethyl-1-tetralone-d(8) (DMT-d(8)) in methylcyclohexane were carried out between 15 and 80 K to analyze their triplet-state hydrogen- and deuterium-transfer reactions. As expected for a decay process displaying a very large isotope effect, emission could be observed only for the deuterated compound. Phosphorescence intensities and decay measurements in the case of DMT-d(8) were analyzed to determine the rates of deuterium transfer which were used to build an Arrhenius plot where tunneling can be readily identified. A temperature-independent triplet state reaction rate (k(D)(QMT) = (5 +/- 1) x 10(2) s(-1)) below 40 K was assigned to quantum mechanical tunneling. Detection of the photoproduct after irradiation at 77 and 15 K was accomplished by fluorescence and by FTIR spectroscopy. Results obtained in this study are in excellent agreement with laser flash photolysis results reported by Al-Soufi et al. (Al-Soufi, W.; Eychmuller, A.; Grellmann, K. H. J. Phys. Chem. 1991, 95, 2022-2026).