Treatment of RuCp[OC(6)H(4)(CH(2)CH = CH(2)-2)-k(1)O: eta(2)C,C'](PPh(3)) (3c) with a Bronsted acid (HX) such as 2-allylphenol results in facile migration of a benzylic proton to the aryloxide, giving the (eta(3) -allyl)ruthenium(II) complex RuCp[CH(2)CHCH(C(6)H(4)OH-2)-eta(3)C,C'C ''](PPh(3)) (4c). Thermodynamic and kinetic studies suggest that 3c associates with acid to give 3c center dot HX, and further addition of HX to 3c center dot HX causes the C-H bond cleavage reaction to give 4c.