Scanning tunneling microscopy was used to study the dissociative adsorption of Br-2 on a Pt(111) surface held at 25 K. At low coverage, Br atoms were observed in correlated pairs with a mean separation of 9.2 +/- 1 Angstrom, indicative of substantial repulsive energy release and transient mobility as the nascent Br atoms separate during dissociative adsorption. An upper bound for the friction coefficient of these hot Br atoms on Pt(lll) was calculated to be eta less than or equal to 2.2 ps(-1). At higher Br-2 exposures, Br atoms were found in chainlike structures, and eventually two-dimensional islands, even though thermal diffusion of Br atoms was quenched at 25 K, The latter observations are consistent with Br-2 adsorption into a transient molecular precursor state and enhanced dissociation in the vicinity of preadsorbed Br atoms.