The singlet excitation energy transfer between pentacene guests in p-terpehnyl was studied as a function of temperature and guest concentration. Transient population gratings were created by exciting the origin of the S-1 <-- S-o transition of pentacene in the O-4 Site of the triclinic p-terphenyl crystal and were probed by Bragg-diffracting a time-delayed pulse of the same frequency. The gratings were observed to decay exponentially with a rate that increases with rising guest concentration and becomes smaller at higher temperatures. No dependence on fringe spacing was found in the range of 8.5-20 mu m. These observations were explained by resonant energy transfer between the O-4 sites and trapping by the other, lower-lying sites of pentacene.