The problem of reaction kinetics in microdisperse systems undergoing collisional exchange is addressed. The migration of reactants is suggested to occur as successive one-particle hopping through fluctuational channels between the droplets during their collisions with temporary merging. The redistribution dynamics within the droplet encounter pair are described using a stochastic approach. The resulting transition probability is calculated as a function of the hopping frequency and the characteristic lifetime of the connecting channels. A hopping mechanism for collisional exchange is employed in analyzing migration-assisted time-resolved luminescence quenching. Applications to fluorescence stopped-flow experiments are also discussed. The model is further extended to include reaction during micellar collisions. An important particular case of an instantaneous reaction is considered, with special regard to concentration jump experiments.