The quality of the pharmaceutical film-forming material (Metolose) is crucial during the tablet-coating process. The aim of this study was to establish a connection between the concentration of the plasticizer (poly-ethylene-glycol, PEG) in the cast film and the free volume. We studied the o-Ps lifetime in a series of Metolose-PEG systems as a function of PEG concentration in the widest available range. The samples were thin films cast from a dispersion of Metolose in aqueous solution of PEG. The found concentration dependence of the free volume in the Metolose-PEG systems is compared with other physical-chemical properties of the samples.