Applying forced Rayleigh scattering to monitor the drying behavior of poly(n-butyl-methacrylate-co-acrylic acid) dispersions (T > T-g + 5 K) via the diffusion of a hydrophobic dye, we find a characteristic length scale dependence of the tracer diffusion coefficient D-app(Lambda) (Lambda = 0.17-10 mum), which allows one to quantitatively describe the transition from a wet, inhomogeneous to a dry, homogeneous polymer latex film within a two-state diffusion model. D-app(Lambda) showed an enhancement of up to 2 orders of magnitude when increasing the length scale. These findings can be quantitatively rationalized within the two-state model assuming Fickian diffusion proceeding slowly in the latex cores and fast diffusion in a heavily plasticized, interfacial phase. From the water content dependence of the model parameters we conclude that the drying process proceeds first by exclusive water loss from the interfacial phase, while the properties of the particle cores remain unchanged. When the Lambda dependence of D-app(Lambda) disappears, water withdraws also from the cores and a homogeneous polymer film forms. Our approach allows one to quantitatively follow property changes in the different compartments of a drying latex dispersion.