The energy profile for the tautomerization reaction of 1,4-dimethylanthrone in the first triplet electronic state obtained through electronic calculations (B3LYP/6-31G(d)) is used to calculate the rate constants for the process at a wide range of energies using a modified RRKM microcanonical statistical formalism that takes into account tunneling. Through partial or total substitution of the hydrogen atoms of the methyl groups by deuterium atoms, it is possible to evaluate different primary and secondary kinetic isotope effects (KIE). These results can be compared with experimental data for these processes taking place in solid matrix at extremely low temperatures (4-50 K). Such a comparison allows us to conclude that the reaction is taking place at energies just slightly below (around 0.5 kcal/mol) the adiabatic potential energy barrier, a result that was previously found for other related molecules so that this mechanism may be extended to the photoenolization of other o-aryl methyl ketones. Analysis of the different factors contributing to the primary and secondary KIEs discloses that at energies not far below the adiabatic barrier, the tunneling effect is not the only factor that accounts for the large KIE but the differences in the energy level distribution upon isotopic substitution may be the. predominant factor at a certain range of negative energies (this is especially so for the case of primary KIE). At positive energies (above the barrier) the levels factor is always the dominant factor in the total KIE. © 2007 American Chemical Society.