This paper presents an experimental method for validating traditional compressor valve theory for unsteady flow conditions. Traditional valve theory considers the flow force acting on the plate and the flow rate as quasi-steady variables. These variables are related via semi-empirical coefficients which are determined by steady flow experiments. The new experimental methodology permitted the simultaneous measurement of instantaneous valve opening, instantaneous volume-flow rate and instantaneous pressure difference across the valve. Results for an oscillating valve (at 1.9 times the valve resonance frequency) show that the gas force is predicted reasonably accurately. However, the flow rate model should be improved in order to predict the observed hysteresis (30 %) and fluctuations in the vena contracta factor.