A means for measuring capacitance of tunnel diodes as a function of forward bias voltage is presented. The method differs from the usual bridge-type measurements in that the parameters are determined while the diode is switching between stable states. This eliminates the difficult requirement of stabilisation in the negative resistance region, and the need to correct for series resistance, inductance and shunt conductance. Unexpected results were obtained which depend on the 'direction' of switching. A qualitative explanation of this behavior is offered which involves the filled or empty condition of traps at the start of switching. © 1964.