Translational diffusion of single perylene diimide molecules in 25 nm thin polymer films was investigated by single molecule widefield fluorescence microscopy. Spatial heterogeneities in single molecule motion were detected and analyzed by a new, quantitative method which draws a comparison of log-Gaussian fits of experimentally determined diffusion coefficient-distributions and diffusion coefficient-distributions from Monte Carlo random walk simulations. Heterogeneities could be observed close to the glass transition temperature, but disappear at ca. 1.1 × T(g). At higher temperatures, heterogeneities do not exist or they average out on the time and length scales of observation. The observed heterogeneities also explain why the dependency of diffusion coefficients on temperature does not follow Vogel-Fulcher-Tammann behavior.