Photophysical methods are used to study the effect of crystallinity on the distribution and mobility of probe molecules in well-characterized polymer films. A combination of steady-state and time-resolved methods indicate that probe molecules are excluded from lamella-like crystalline zones in the polymer systems. This effect tends to concentrate the probes into the amorphous zones that exist between crystallites. Using the pyrene-perylene donor acceptor pair as a "molecular ruler", it is shown by Forster analysis that the amorphous zones form a network structure throughout the polymer film. Mobility within this region is greatly reduced from that in solution and is very size selective, as indicated by the quenching rate constants of the pyrene triplet state by oxygen, azulene, and ferrocene. A determination of the oxygen concentration, by comparing the permeability to diffusibility within the film, indicates that crystalline regions are impermeable to gas molecules. The mobility of the guest molecule in the film is orders of magnitude greater at temperatures above the glass transition temperature T-g, compared to the mobility below T-g. Simple calculations using free volume theory can account for the observed rate data.