Clouds are of crucial importance for a planetary climate, because they store atmospheric volatiles, and because they scatter and absorb incident starlight and absorb, emit and scatter thermal radiation. Consequently, the detection and characterization of clouds on a planet can provide us with a wealth of information on the conditions on the surface. Here, we present numerically simulated flux and polarization spectra, from 0.3 ensuremathμm to 1.0 ensuremathμm, of starlight reflected by Earth-like exoplanets that are covered by horizontally homogeneous water clouds, for different cloud altitudes and particle sizes. Our results show that the degree of polarization P is sensitive to the particle size, in particular at phase angles between 30° and ensuremath∼50° and around 90°, and to the cloud top altitude, in particular at wavelengths between 0.35 ensuremathμm and 0.7 ensuremathμm. The information in P should be easier to retrieve than that in F.