The room-temperature reaction of Cl-2 with GaAs(001)-4x6, -c(2x8), and -c(4x4) surfaces is studied with synchrotron soft x-ray photoelectron spectroscopy. The chemical composition of the reacted surfaces is found to depend on the stoichiometry of the starting surface. In all cases, the reaction occurs stepwise, with Ga and As monochlorides formed prior to the dichlorides. The Ga-rich surface is initially more reactive than either of the As-rich surfaces and it forms more GaCl than the As-rich surfaces, which instead form more AsCl. The sticking coefficient for chlorine on GaAs(001) decays exponentially with coverage. A contribution from Cl atoms comprising the surface dichlorides is identified in the Cl 2p core-level spectra.