The selectivity of photorearrangements of five I-naphthyl and 2-naphthyl acylates has been investigated in three unstretched and stretched polyethylene films of different crystallinities and in isotropic solutions. The influence of variables such as size and position of the aryl groups of the esters, degree of crystallinity, free volume, and unstretched/stretched state of the films has been explored. The orthopositronium accessible free volume sites in the undoped unstretched and stretched polyethylenes have been measured by positron annihilation lifetime spectroscopy. These void free volumes are much smaller than the van der Waals volumes of the naphthyl molecules under investigation. The naphthyl eaters inside polyethylene cavities act as templates for the formation of their photoproducts. Long alkyl chains on naphthyl myristates not only affect the shape anisotropy but also induce large van der Waals interactions with the walls of reaction cavities. Stretching enhances the templating effect and strengthens the van der Waals attractions with cavity walls in the case of naphthyl myristates, thus inducing marked increases in reaction selectivities in polyethylene films. Somewhat surprisingly, there is no correlation between void free volume of a host polyethylene film and selectivity of photoreactions of a guest naphthyl eater.