Branching between reaction channels in activated systems is often observed to vary with changes in the potential energy surface as Delta ln(k(1)/k(2)) proportional to Delta(Delta E(1)(double dagger) - Delta E(2)(double dagger)). RRKM calculations demonstrate that in many, but not all, cases the log-linear relationship accurately describes branching in nonthermal distributions of reactants with energies well above the threshold for reaction. The origin of this relationship and conditions necessary for its validity can be understood in terms of quantum RRK theory.