A new method for preparing thin films for use in beta-spectrometry and 4π-counting is described. These films, made of polyvinylchloride-acetate copolymer are readily prepared with superficial densities as low as 1 μgm. per cm.2The films have good tensile strength and show excellent resistance to acids, alkalies, and many organic reagents. The thickness of the films may be determined (a) gravimetrically, (b) radiometrically, and (c) optically either by transmission or reflection. The films can be made conducting by distilling a thin gold layer on to them. The thickness of the gold layer can be determined spectrophotometrically.