Time-resolved measurements of photoinduced reactions reveal that many ultrafast reactions in the femto- to picosecond time scale are nonexponential. In this article we provide several examples of reactions that exhibit a nonexponential rate. We explain the wide range of the nonexponential reaction by the lack of time separation between tau(s), the characteristic fast equilibration time of the population in the reactant potential well, and the longer time tau(e), the characteristic time to cross the energy barrier between the reactant and the product.