Potential energy surfaces of the rhodium(I)-catalyzed olefin hydroboration reactions, RhCl(P(H)3)(2) + HB(OH)(2) + C2H4 --> RhCl(PH3)(2) + C2H5B(OH)(2) (1) and RhCl(PH3)(2) + HBO2(CH2)(3) + C2H4 --> RhCl(PH3)(2) + C2H5BO2(CH2)(3) (2), have been studied by using ab initio molecular orbital method at the MP2/ECP+DZ level. The following mechanisms have been considered : (I) oxidative addition of a B-K bond to the metal center, followed by olefin coordination to the complex in various positions without dissociation of PH3 group, further followed by insertion of olefin into either M-H or M-B bond and reductive elimination of B-C or B-H bond, respectively and (II) coordination of olefin to the metal center, followed by "sigma-bond metathesis" involving coordination of borane and simultaneous cleavage of the M-C and B-H bonds with formation of the M-B and H-C or M-H and B-C bonds. For both reactions, the most favorable mechanism is shown to involve oxidative addition of borane to the catalyst and coordination of C2H4 to the complex between B and H ligands trans to Cl, followed by insertion of C=C into the Rh-B bond. The reactions are completed by dehydrogenative reductive elimination of C(2)H(5)BR which is calculated to be the rate determining step and to have the barriers of 22.4 and 20.8 kcal/mol for eqs 1 and 2, respectively. Other competitive mechanisms involve as the rate-controlling step the "sigma-bond metathesis" to break B-H and to form M-H and B-C bonds after formation of the RhCl(PH3)(2)(C2H4) complex, with the barrier of 23.9 kcal/mol for reaction 1.