Phosphorescence intensities and lifetimes of 1,4-dimethylanthrone (1,4-MAT) and 1,4-dimethylanthrone-d(8) (1,4-DMAT) were measured to determine the involvement of activated and quantum mechanical tunneling mechanisms (QMT) in their hydrogen and deuterium atom transfer reactions. The thermal-dependence of the radiative and thermal decay of the anthrone chromophore and the effect of methyl substitution were evaluated by using anthrone (AT), 2,3-dimethylanthrone, (2,3-MAT), and 10,10-dimethylanthrone (10,10-MAT). Measurements were carried out in methylcyclohexane (MCH) glasses between 18 and 80 K. The unreactive molecules AT, 2,3-MAT, and 10,10-MAT present phosphorescence parameters typical of diarylketones with (3)n,pi* configurations and show a relatively small temperature dependence changing from monoexponential at 77 K to nonexponential at the lowest temperature values. The phosphorescence intensity from 1,4-DMAT was extremely weak and highly temperature-dependent. In contrast, no phosphorescence was detected in 1,4-MAT at all temperatures studied. Differences between deuterio and protio compounds were analyzed in terms of a large isotope effect on the hydrogen atom transfer reaction. A quantum mechanical tunneling mechanism was confirmed from nonlinear Arrhenius plots constructed with the average deuterium transfer rates of 1,4-DMAT. A temperature-independent quantum mechanical tunneling reaction with a rate of 2 x 10(3) s(-1) was calculated between 30 and 18 K. The involvement of reaction was confirmed by accumulation and detection of the photoenol product in ethanol glasses at all the temperatures studied. Changes in phosphorescence intensity observed even under conditions where the triplet lifetimes remain constant (18-30 K) were analyzed in terms of an avoided crossing mechanism predicted by orbital and state symmetry considerations.