Branching between reaction channels in activated systems is oftenobserved to vary with changes in thepotential energy surface asΔln(k1/k2) ∝Δ(Δ−Δ).RRKM calculations demonstrate that in many, butnot all, cases the log−linear relationship accurately describesbranching in nonthermal distributions of reactantswith energies well above the threshold for reaction. The origin ofthis relationship and conditions necessaryfor its validity can be understood in terms of quantum RRKtheory.