An instrumentation technique for the measurement of frontal instability of advancing oil and water fronts into core materials at reservoir conditions has been developed. The technique uses parallel open grid electrodes placed in good electrical contact within the confined core to measure conductance and capacitance. Also associated with each electrode is a thermocouple to give information associated with thermal fronts. Instabilities are identified by analysis of the changing capacitance and conductance induced by the movement of the front and its instabilities. This technique permits measurement of the velocity of the fronts, the fingers resulting from instability in the frontal advance, and the area and length of these fingers. A number of cases were investigated; these included brine front injection into a virgin core and oil displacement of a brine saturated silica core. As expected, in a relatively stable displacement the fingers moved at relatively slower velocities than the fronts, but not at a constant differential velocity. Often, considerable extension was observed to be reduced only at another stage of the displacement. This general technique can be adapted to core and laboratory-scaled physical models where information on frontal stability important.