The wetting of ultrathin films of polystyrene on the hydrophilic surfaces of mica and silicon oxide was studied by atomic force microscopy. After annealing, the surfaces were covered with a homogeneous, continuous polystyrene film of roughly 1 nm thickness. On top of this film, polystyrene droplets with microscopic contact angles of 7degrees-16degrees were observed. After exposure to an oversaturated water vapor, the continuous polystyrene film disintegrates and dewets the surfaces. Polystyrene structures on silicon oxide indicate a homogeneous dewetting process starting from few nucleation sites. On mica the density of nucleation sites for water is much higher and the polystyrene dewets the surface in an inhomogeneous process. The structural changes observed imply that ultrathin polystyrene films are highly mobile in the presence of water.