The oxorhenium(V) dimer {MeReO(edt)}(2) (1; where edt = 1,2-ethanedithiolate) catalyzes S atom transfer from thiiranes to triarylphosphines and triarylarsines. Despite the fact that phosphines are more nucleophilic than arsines, phosphines are less effective because they rapidly convert the dimer catalyst to the much less reactive catalyst [MeReO(edt)(PAr3)] (2). With AsAr3, which does not yield the monomer, the rate law is given by v = k[thiirane][1], independent of the arsine concentration. The values of k at 25.0 degrees C in CDCl3 are 5.58 +/- 0.08 L mol(-1) s(-1) for cyclohexene sulfide and ca. 2 L mol(-1) s(-1) for propylene sulfide. The activation parameters for cyclohexene sulfide are Delta H-double dagger = 10.0 +/-0.9 kcal mol(-1) and Delta S-double dagger = 21 +/- 3 cal K-1 mol(-1). Arsine enters the catalytic cycle after the rate-controlling release of alkene, undergoing a reaction with the Re-VII(O)(S) intermediate that is so rapid in comparison that it cannot be studied directly. The use of a kinetic competition method provided relative rate constants and a Hammett reaction constant, rho = -1.0. Computations showed that there is little thermodynamic selectivity for arsine attack at O or S of the intermediate. There is, however, a large kinetic selectivity in favor of Ar3AsS formation: the calculated values of Delta H-double dagger for attack of AsAr3 at Re=O vs Re=S in Re-VII(O)(S) are 23.2 and 1.1 kcal mol(-1), respectively.