In the present work it is demonstrated how dynamical measurements of the work function may be used to investigate the growth of thin epitaxial metal films. A special setup of a basically old technique-a redesign of a Kelvin probe-is applied to monitor work function changes not only during gas adsorption but also during metal deposition with high precision. Such dynamical work function change measurements clearly demonstrate the layer-wise growth of thin Cu films on an oxygen precovered Ru(0001) surface. This is revealed by the observation of multiple DELTAPHI-oscillations with a period corresponding to the deposition of a single Cu(111) layer. Furthermore the role of the oxygen as a surfactant for layer-by-layer growth is concluded from the fact that under similar conditions such oscillations have never been observed during Cu evaporation on the clean Ru(0001).