Rhodium(III) tetra(p-sulfonato phenyl) porphyrin ((TSPP)Rh) aquo and hydroxo complexes react with a series of olefins in water to form beta-hydroxyalkyl complexes. Addition reactions of (TSPP)Rh-OH to unactivated terminal alkenes invariably occur with both kinetic and thermodynamic preferences to place rhodium on the terminal carbon to form (TSPP)Rh-CH2CH(OH)R complexes. Acrylic and styrenic olefins initially react to place rhodium on the terminal carbon to form Rh-CH2CH(OH)X as the kinetically preferred isomer but subsequently proceed to an equilibrium distribution of regioisomers where Rh-CH(CH2OH)X is the predominant thermodynamic product. Equilibrium constants for reactions of the diaquo rhodium(III) compound ([(TSPP)Rh-III(H2O)2](-3)) in water with a series of terminal olefins that form beta-hydroxyalkyl complexes were directly evaluated and used in deriving thermodynamic values for addition of the Rh-OH unit to olefins. The Delta G degrees for reactions of the Rh-OH unit with olefins in water is approximately 3 kcal mol(-1) less favorable than the comparable Rh-H reactions in water. Comparisons of the regioisomers and thermodynamics for addition reactions of olefins with Rh-H and Rh-OH units in water are presented and discussed.