Lead vapor trapped at 20°K in Ar, Kr, Xe, SF6, and in mixtures of these matrix gases showed an atomic absorption line with no multiplet structure and molecular bands of Pb2. Warming the matrices above 20°K shifted the atomic line reversibly to the red while the molecular features did not shift. Bulk diffusion of isolated Pb atoms to produce Pb2 did not take place in Xe even after 2 h at 50°K. However, surface diffusion of Pb atoms to form Pb2 can take place during the condensation process on a 20°K window under conditions of high heat load or poor thermal contact that cause high surface temperature.