In this article, we describe an experiment for the undergraduate physical chemistry laboratory in which students measure the compressibility factor of two gases, helium and carbon dioxide, as a function of pressure at constant temperature. The experimental apparatus is relatively inexpensive to construct and is described and diagrammed in detail. Students analyze their data to determine values for the second virial coefficient for each gas, which may then be compared with the literature. The experiment demonstrates the nonideality of real gases. (Contains 2 tables and 2 figures.)