In the experiments described in this paper, micromachined silicon post surfaces were coated with thin films of a polymer for which the contact angle on the smooth material was around 70 degrees. Drops wetted these surfaces in the Wenzel or "penetration" mode. We have determined the advancing and receding angles as a function of the roughness geometry and quantitatively compared our results to the contact angles predicted by Wenzel's model. We discuss reasons for discrepancies and propose a model for the motion of the meniscus where we take into account the precise shape of the liquid front during its movement through the post structure.