Hydrogen–deuterium exchange (H/D exchange) is a method commonly used for studying catalytic activation of C–H(D) bonds by transition metal complexes. In this study, a series of additives were studied for H/D exchange of toluene-d₈ with acetic acid (HOAc) using (^RPNP)Rh(X) complexes (R = phosphine substituents including cyclohexyl, isopropyl, and tert-butyl; X = trifluoroacetate or acetate) as the precatalysts. Cu(OAc)₂ and AgOAc additives were found to benefit Rh-mediated C–H(D) activation of toluene with meta–para selectivity by facilitating the conversion to active (^RPNP)Rh species and stabilizing the Rh catalysts from decomposition to inactive Rh(s). In contrast, nonoxidizing Lewis acid additives, such as B(OMe)₃ or NaOAc, were not effective at facilitating Rh-catalyzed toluene C–H activation. The complexes (^RPNP)Rh^III(H)(X)₂ and [(^RPNP)Rh^I(CO)][X] (X = TFA or OAc) were found to be intermediates of the catalytic the H/D exchange.