Light- and voltage-induced changes in the microwave reflectivity of semiconductors can be used to study the kinetics and mechanisms of electron transfer at semiconductor I electrolyte interfaces. The theory of the method is developed and illustrated by numerical calculations of the steady-state microwave response for low-doped silicon. The results define the range of rate constants that should be experimentally accessible using microwave reflectivity methods. The time and frequency responses of light-induced microwave reflectivity changes are considered, and it is shown that they can be used to derive values of electron transfer and recombination rate constants.