The diffusion of deuterium-labeled atactic polystyrene (dPS) from the surface of a precrystallized polydisperse, isotactic polystyrene (iPS) matrix was measured using dynamic secondary ion mass spectroscopy (DSIMS). The detailed depth profiles indicated two types of mass transport into the iPS matrix. A layer near the surface of the film was observed to fit a Gaussian function whose prefactor decreased with diffusion time but whose width increased only very slowly with diffusion time. An underlying profile was observed to correspond to Fickian diffusion from a constant composition, C-0, of dPS at the interface with the surface layer into the underlying iPS. The concentration, C-0, decreased with increasing dPS degree of polymerization, N, and appears to be the saturation concentration at the surface of the iPS layer. The tracer diffusion coefficient, D, extracted from the underlying diffusion profile decreased as N-1. This N dependence is suggestive of entropic barrier type diffusion.