The decrease of optical thickness of drying latex coatings is measured interferometrically. Through data processing film thickness and polymer volume fraction are obtained, which are a measure of particle spacing and deformation. The applied criterion for film formation is the attainment of a dense polymeric material. In this sense the method can be used to check film formation under a variety of conditions. The kinetics of film formation is monitored as well. Since the method is sensitive to changes in optical properties, different modes of drying can be distinguished. The solids content determined by this method is not averaged over the whole film but represents the exact state of drying at the measured spot. Another advantage is that due to the simple setup of the experiment, it works under a broad variety of conditions and without restrictions regarding the nature of the latex. Also, simultaneous measurements at two angles of incidence allow refractive index independent data interpretation. Drying of different dispersions, both film forming and non-film forming under the given conditions, has been investigated, demonstrating the potential of the technique.