How a molecule impinging from an ambient phase attaches to and reacts with a preadsorbed reactant was studied by scanning tunneling microscopy (STM). A carboxylate (RCOO-) chemisorbed on a TiO2(110) surface was exchanged by another carboxylate (R'COO-) via an S(N)2-like mechanism when the RCOO--covered TiO2 surface was exposed to R'COOH vapor at room temperature or below. An R'COOH impinging from the gas phase was temporarily bound to a preadsorbed RCOO- yielding a bimolecular intermediate. The trapped R'COOH was inserted into the monolayer and dissociated to produce an adsorbed R'COO-. One of the carboxylates (RCOO- or R'COO-) was then pushed out of the monolayer to restore the intermediate and released to the vapor phase. The individual reaction steps were resolved using time-lapse imaging.