Interdiffusion of polymer chains between latex particles is a prerequisite for the development of good mechanical strength and homogeneity in a latex film. This process may be retarded in carboxylated latices if the particles are surrounded by a hard cell wall consisting of ionoc groups on the particle surface. The presence of an ionic cell wall can be indirectly detected by atomic force microscopy (AFM) because surfactant migration to the film/air interface is retarded compared with a non-ionic case. In this paper we have used force modulation atomic force microscopy to directly probe the relative polymer density across the film surface during annealing thereby qualitatively monitoring the interdiffusion process. The applicability of this method to study polymer interdiffusion will be discussed.