A method for measuring the thickness of thin foils is described. The chamber was designed so that foil thicknesses from 5.0 to 0.5 mg/cm 2 could be determined by measuring the energy loss of Po 210 α-particles passing through matter. Thicknesses from 0.5 mg/cm 2 to 5.0 μg/cm 2 or less could be determined by comparing the equivalent thicknesses of air and foil material. Calibration curves are given for Al, Cr, Ni and Sn. The accuracy of measurements is better than 10%.