The backscattering of 2500-keV alpha particles is used to measure the densities of amorphous Si films. The density of amorphous Si is found to be inversely proportional to the electron spin resonance signals. The extrapolated density at zero spins is equal to 1.01 ± 0.02 times that of crystalline Si. The highest density achieved in a real amorphous film was 0.97 ± 0.02 times the crystalline density. © 1972 The American Institute of Physics.