We study global decay of the bimolecular reaction A+B→P as c(t)∼t−α in a nonlinear transitional flow. A relationship is established between the decay exponent α, and a modified Kolmogorov–Sinai entropy, hr. We find that for dynamic conditions which induce relatively strong mixing, the decay exponent is α∝ln ψ−Bhr with B being a characteristic reactive mix-down time for the system, and ψ is a space–time scaling parameter. Dynamic conditions which imply weak mixing lead to a degenerate dependence of α on hr. The proposed relationship between α on hr should be a useful link between the dynamical evolution of the flow field and reaction kinetics in vortex dominated flows.