The reduction of O-2 to H2O mediated by a series of electronically varied rhodium hydride complexes of the form cis,trans-(RhCl2H)-Cl-III(CNAd)(P(4-X-C6H4)(3))(2) (2) (CNAd = 1-adamantylisocyanide; X = F (2a), Cl (2b), Me (2c), OMe (2d)) was examined through synthetic and kinetic studies. Rhodium(III) hydride 2 reacts with O-2 to afford H2O with concomitant generation of trans-(RhCl3)-Cl-III(CNAd)(P(4-X-C6H4)(3))(2) (3). Kinetic studies of the reaction of the hydride complex 2 with O-2 in the presence of HCl revealed a two-term rate law consistent with an HX reductive elimination (HXRE) mechanism, where O-2 binds to a rhodium(I) metal center and generates an eta(2)-peroxo complex intermediate, trans-(RhCl)-Cl-III(CNAd)(eta(2)-O-2)(P(4-X-C6H4)(3))(2) (4), and a hydrogen-atom abstraction (HAA) mechanism, which entails the direct reaction of O-2 with the hydride. Experimental data reveal that the rate of reduction of O-2 to H2O is enhanced by electron-withdrawing phosphine ligands. Complex 4 was independently prepared by the addition of O-2 to trans-(RhCl)-Cl-I(CNAd)(P(4-X-C6H4)(3))(2) (1). The reactivity of 4 toward HCl reveals that such peroxo complexes are plausible intermediates in the reduction of O-2 to H2O. These results show that the given series of electronically varied rhodium(III) hydride complexes facilitate the reduction of O-2 to H2O according to a two-term rate law comprising HXRE and HAA pathways and that the relative rates of these two pathways, which can occur simultaneously and competitively, can be systematically modulated by variation of the electronic properties of the ancillary ligand set.