A molecular beam at temperatureTGwas impinged onto the surface of a copper block held at temperatureTS. The fraction of molecules reflected was 1−γ, where γ is the condensation coefficient. Of the reflected molecules, a fixed fraction was condensed on the surface of a quartz crystal microbalance. Precision measurements of the mass flux onto the microbalance gave a direct determination of the rate of reflection of molecules from the surface of the copper block. This information, combined with the known flux of molecules onto the copper block, gave high precision (∼0.01%) values of γ. ForCO2condensing on a film of solidCO2, γ>0.96 whenTS≤83 KandTS=274 K. Water molecules condensing on water ice films give γ≥0.99 whenTS≤150 KandTG=273 K. Values of γ forCO2were found to depend onTS,TGand the molecular beam intensityṅCB. The range through whichṅCBvaried was1.5×1014to1.5×1016 molecules cm−2⋅sec−1.