The isotope effects on bifurcating reaction paths have been studied for two reactions, H3CO --> H2COH and HF+C2H4--> C2H5F, both of which have a valley-ridge inflection (VRI) point related to nontotally symmetric directions on the symmetry-conserved intrinsic reaction coordinate (IRC). As the result of the VRI occurrence, the IRC connects a reactant minimum, the transition state, and another first-order saddle point. The deuterium substitution of one or two hydrogen atoms in molecular systems reduces the symmetry of the IRC, resulting in the direct linkage of the transition state and a product minimum through the IRC. In the above two reactions with deuterium substitution, it is found that the VRI occurs relative to the totally symmetric directions. Analyses of the steepest descent paths that start from a zero-point energy region around the VRI point demonstrate that those paths bifurcate toward two different product minima.