An apparatus of the Burnett type has been built with which to measure accurately the second virial coefficients of gases at low temperatures. The principal feature is a mercury manometer with which pressures up to 1 bar can be measured with an accuracy of 2 × 10–6 bar. The second virial coefficients B of argon (80–190°K) and of krypton (110-225°K) have been measured with absolute accuracies of from 0.001 to 0.01 m3 kmol–1, and are discussed in relation to the pair potential of the intermolecular forces. The rapid fall of B at low temperatures implies, in each case, a deeper well in the potential than has been proposed heretofore.