The heat capacities of O2, N2, CO, and NO have been measured in the temperature range from 4.2 °K to about 0.8 °K. No anomalous behavior was found. The results were fitted to the Debye expression (assuming 3 degrees of freedom per molecule) to yield the following values for Θ0: O2, 104.5 ± 1.0 °K; N2, 83.5 ± 1.0 °K; CO, 103.0 ± 1.0 °K; NO, 122 ± 2 °K. The results are discussed in terms of the residual entropies of CO and NO.