Condensation coefficients were measured by impinging a known flux of gas from a molecular beam onto the surface of a quartz crystal microbalance. The rate of change of the quartz crystal's resonance frequency was used to calculate the condensation rate. The condensation coefficient, which is the ratio of the condensation rate to the impingement rate, was found to be ≥0.95 for Ar, Kr, Xe, andCO2when surface temperatures are less than 26, 37, 47, and 55 K, respectively, and the gas temperatures are ≤300 K for Ar, Kr, and Xe and ≤200 K forCO2. The condensation coefficients of Ar on Kr and Xe films, Kr on Ar, and Xe on Kr were also determined as a function of surface coverage from about 0.01 to 10 atomic layers.