Return Voltage Measurements are an efficient tool to measure the dielectric properties of an insulating system and thus to characterize the degradation of cellulose-oil insulations. They are widely used for the diagnosis of power cables. As a consequence of misinterpretations of the results in the past, the use Return Voltage Measurements for power transformers has been reduced. Nevertheless - performing a physically meaningful interpretation of the measured curves - the application on power transformers is also very promising, especially because the geometry of the insulation system in a power transformer is closer to the conditions described by the Maxwell model for two dielectrics in series. Return Voltage curves for new or not severely aged transformers fit very well the type and shape of the voltage curve predicted by the Maxwell equivalent circuit. Ageing and degradation phenomena in the oil and the paper lead to a deviation from this behaviour.