The principles and applications of microwave reflectivity measurements in semiconductor electrochemistry are reviewed and illustrated by theoretical calculations and experimental examples. The microwave response of the illuminated p-Si \ NH4F junction has been studied under depletion conditions and related to the calculated concentration profiles of electrons and holes. Time- and frequency-resolved measurements have been used to follow the interfacial transfer of photogenerated electrons to protons in solution. The rate constant for interfacial electron transfer is very small, probably reflecting the absence of low energy sites for stabilisation of the intermediate, in the two-electron reduction of H+ to H-2. The time dependent measurements provide evidence for reversible hydrogen absorption into the surface region of the silicon. Potential modulated microwave reflectivity has been used to study the behaviour of p-type silicon in fluoride solutions under depletion and accumulation conditions in the dark. Under depletion conditions, the time-resolved and periodic microwave responses are related to the changes in the width of the space charge region (SCR), and the sensitivity factor that relates the normalised reflectivity changes to changes in carrier concentrations can be obtained by comparison of the microwave response with the potential dependent space charge capacitance. Under accumulation conditions, dissolution of the p-Si occurs, resulting in porous silicon formation or electropolishing depending on the applied potential. In this case, the microwave response gives information about the potential distribution across the Si I (oxide) I solution system.