Dielectric multilayer coatings of SiO2 and Si3N4 prepared by RF diode sputtering show very good environmental stability which surpasses that of conventional vapor deposited coatings. It is generally assumed that sputtered films have a higher packing density which makes them less susceptible to water penetration. Transmission electron micrographs of cross-section replicas of these multilayer structures show a columnar microstructure of the individual Si3N4 films which is qualitatively denser than that of other high index films made by electron beam deposition. Hence, some porosity may be still assumed for the sputtered films, and therefore other mechanisms to make them less humidity susceptible than just desiccation. For instance, it has been found that the wetting properties of dielectric thin films not only depend on their material but also, to a minor extent though, on the method of how they were deposited. Another possible explanation for improved humidity stability is that the plasma discharge in the sputtering process may polymerize residual hydrocarbons or silicone in the vacuum chamber to form hydrophobic thin films in between the dielectric layers and also on their inner surfaces, so that the voids between the columns in the films become less able to absorb water. First results on both contact angle measurements to assess the wettability of coating surfaces and on electron spectroscopic imaging for detecting presumed hydrocarbon contamination are presented.