Optical coatings based on multilayers are prepared. The dependence of the refractive index of SiO2, TiO2, HfO2, and ZrO2 on the conditions during thin film evaporation (e.g., oxygen partial pressure, substrate temperature) and upon post-annealing are investigated in detail. For coatings to be stable under intensive laser irradiation, post-heating of the evaporated films is necessary. The correlation between mass density and refractive index is analysed on the basis of a model comprising compact grains (i.e., amorphous or crystalline) and pores filled with air or water. The properties of the whole film are discussed on the basis of an effective-medium model, whereas the properties of a single grain are derived from a general refractivity formula valid for homogeneously distributed dipoles.