We describe an experimental method for in situ resistivity measurements during the electrochemical hydrogen loading of thin, switchable metal hydride films. Using an oxygen-free electrolyte we are able to measure the hydrogen concentration in the films quantitatively and to determine the pressure-composition isotherms and the hydrogen concentration dependence of the resistivity. Furthermore, the optical properties can be investigated by simultaneous transmission and reflection spectroscopy. The power of these in situ measurements is discussed on the basis of results obtained on YHx films; possibilities for additional experiments are briefly described.