Electrochemical microprobes for monitoring of the electrochemical conditions inside recesses are needed to better understand how localized corrosion and other charge-transfer processes occur in confined spaces. The electrode potential distribution can be routinely measured in cavities of greater than 100 mum opening dimension, but the ability to measure the concentrations of chemical species and their change with time in the presence of potential gradients is only recently becoming possible. Microprobes for potential, pH, and chloride ion will be described, and results with these sensors will be presented and discussed for creviced iron samples in aqueous electrolytes. Results reveal that these species change in concentration and distribution with time, with the greatest ionic concentrations corresponding to locations of highest metal dissolution rate on the crevice wall. These new physical chemistry insights provide a better understanding of the passive-to-active behavior in crevices.