We report a study of the photodimerization of properly arranged anthracene pairs generated by photolysis in a dianthracene crystal at 2 K. We monitor the progress of the photodimerization reaction by measuring the fluorescence lifetime of the pair excimer state, and we use pressure as an empirical parameter. Dimerization is too fast to monitor beyond 6 kbar for the normal (protonated) anthracene pairs, and beyond 10 kbar for the pedeutero sample. The results are interpreted as dimerization through a tunneling mechanism, although evidence of a photophysical retardation was observed. Pressure enhancement of the fluorescence decay rate is exponential. The pressure coefficient for rate enhancement is 0.203 (0.010) natural log units per kbar for the normal sample, and 0.2576 (0.0065) for the perdeuterated sample, respectively (with the standard deviation of the mean given in parentheses). The reaction may be formally construed as nanosecond tunneling of a very heavy particle. The origin of the "reverse" deuterium isotope effect is discussed.