Chlorine adsorption on Ag(100) saturates at (1/2) monolayer (ML) in a c(2x2) structure. Desorption occurs above 650 K without disordering. The sticking coefficient remains constant to within 0.05 ML of saturation, consistent with the existence of a highly mobile precursor to adsorption. The c(2x2) structure forms at a critical coverage of 0.394+-0.007 ML at room temperature. This is slightly higher than the critical coverage of the hard-square model (0.368 ML), suggesting the influence of further-neighbor interactions.