N,N'-Dimethylperylenetetracarboxylic acid diimide (Me-PTCDI) thin films were prepared by vapor deposition technique. The photogeneration and radiative singlet and nonradiative tripler exciton recombination were characterized, utilizing absorption, photoluminescence, and optically detected magnetic resonance (ODMR) spectroscopy. The results suggest the creation of molecular dimers or larger aggregates. The latter effect gives rise to the creation of split singlet exciton states and enhancement of population in the monomer triplet exciton state. The theoretical simulation of the ODMR signal suggests that the dimers are partially oriented with respect to the substrate. They are distributed around the parallel configuration, with a tilt angle of +/-36 degrees.