The reaction of Br-2 with a Cu surface vicinal to (100) was studied by recording scanning tunneling microscopy (STM) movies during exposure to Br-2. Initial exposure to Br-2 resulted in the formation of a chemisorbed layer of Br atoms with a c(2 X 2) structure on the (100) terraces. The STM movies showed that, as the chemisorbed layer saturated, thermal fluctuations in the step positions slowed, and the steps aligned parallel to:the [001] and [010] directions. Further exposure to Br-2 resulted in the formation of CuBr; the STM movies showed that the Cu atoms were consumed from the steps with the reaction proceeding predominantly perpendicular to the step edges. Channels created in the step edges indicated that the reactivity of the steps was not uniform. The CuBr formed by the reaction did not accumulate at the sites where the Cu atoms were consumed; rather, large two-dimensional CuBr islands were observed at seemingly random locations. This indicated that CuBr formed at the step edges was mobile and that the CuBr islands formed by nucleation of diffusing CuBr species. The CuBr islands spanned many steps suggesting that the diffusing CuBr can cross step edges. Atomic-resolution images of the CuBr islands revealed hexagonal symmetry consistent with CuBr(111) parallel to the surface.