Insituinterfacial impedance measurements are used to study the effects of moisture at 80 °C on the interface between oxidized silicon and a diglycidyl ether of bisphenol A epoxy cured with diethylene triamine. Bulk impedance measurements follow Randles behavior; conduction in the bulk epoxy is ionic, and is diffusion controlled at low frequencies. An additional conduction process between interfacial electrodes is observed even though a water layer does not condense in the interface. Through comparative use of linear network models, it is found that the ‘‘interfacial’’ conduction path represents distributed bulk conduction in the epoxy with displacement current leakage into the conductive silicon substrate through an interfacial capacitance. Both electrochemical and bulk coating properties jump at humidities near 70%, indicating greatly increased ionic mobility, coating permittivity, and interfacial capacitances. The permittivity and ionic mobility behavior suggest the formation of large water‐swollen demains or highly elongated water clusters near the property jump threshold. Because humidity effects are resolved into bulk and interfacial components, interfacial impedance measurements appear to have great utility for theinsitustudy of transport and electrochemical properties of interfaces and coatings during environmental exposure.