Student interpretations of the equation for the first law of thermodynamics, [delta]U = q + w, an expression defining work done on or by a gas, w = -[image omitted]PdV, and an expression defining heat, q = [image omitted]C[subscript v]dT were investigated through a multiple-choice survey, a free-response written survey, and interviews. The students examined had completed an undergraduate physical chemistry class in which they studied thermodynamics extensively and used these equations frequently to solve problems. The survey and interview results suggest that many students were unable to distinguish the concepts represented in the first law equation from concepts represented in the equations for work and heat. Students expressed a variety of physical interpretations for these fundamental equations. Some common misinterpretations were identified. These results suggest that far more attention needs to be placed on helping students learn the physical meaning of equations in thermodynamics courses. (Contains 3 tables.)