The objective of this research is to experimentally characterize the spatial distribution of an additive across the thickness of a single-layer polymer film. Concentration-distribution data are then used to quantify the amount of additive that diffuses from the bulk of the film to the film surfaces as a function of time. These results are important because a film's surface property, such as coefficient of friction, depends on the additive surface concentration. The concentration profiles of an erucamide slip agent in linear low-density polyethylene (LLDPE) films were measured with Fourier transform infrared microspectroscopy. This paper describes in detail the procedures used to microtome specimens and map the concentration profiles for the erucamide-LLDPE system. Data show that the additive concentration profile is uniform across the thickness of the film immediately after it has been formed. After aging at 60 °C for 4.5 h (not yet to equilibrium), approximately 40% of the additive accumulates at the film surfaces.