An oxorhenium(V) dimer, {MeReO(mtp)}(2), D, where mtpH(2) is 2-(mercaptomethyl)thiophenol, catalyzes oxygen atom transfer reaction from methyl phenyl sulfoxide to triarylphosphines. Kinetic studies in benzene-d(6) at 23degreesC indicate that the reaction takes place through the formation of an adduct between D and sulfoxide. The equilibrium constants, K-DL, for adduct formation were determined by spectrophotometric titration, and the values of K-DL for MeS(O)C6H4-4-R were obtained as 14.1(2), 5.7(l), and 2.1(1) for R = Me, H, and Br, respectively. Following sulfoxide binding, oxygen atom transfer occurs with either internal or external nucleophilic assistance. Because {MeReO(mtp)}(2) is a much more reactive catalyst than its monomerized form, MeReO(mtp)PPh3, loss of the active catalyst during the time course of the reaction must be taken into account as a part of the kinetic analysis. As it happens, sulfoxide catalyzes monomerization. Monomerization by triarylphosphines was also studied in the presence of sulfoxide, and a mechanism for that reaction was also proposed. Both the phosphine-assisted monomerization and the phosphine-assisted pathway for oxygen atom transfer involve transition states with ternary components, D, sulfoxide, and phosphine, which we suggest are structural isomers of one another.