The deposition of ZnO thin films by atomic layer epitaxy (ALE) from diethylzinc and water precursors is studied for the first rime by an in-situ quartz crystal microbalance technique. Quantitative measurement of the growth with a resolution down to the monolayer level is demonstrated. Influence of temperature, pulse lengths and substrate characteristics has been studied. The site saturation regime (ALE window) is between 100 degrees C and 160 degrees C and corresponds to the growth of one (100) monolayer with possible effects of roughness and surface reconstruction. Effect of nucleation and coalescence has been clearly evidenced both on foreign substrates and more surprisingly on ZnO substrates depending of the duration of the rest period. The analysis of the mass variation during individual cycles raises some questions about the growth mechanism.