The kinetics of film formation of emulsifier-free monodisperse poly(n-butyl methacrylate) (PBMA) latex in the presence of post-added poly(styrene/alpha-methylstyrene/acrylic acid) (SAA), an example of alkali soluble resin (ASR), was followed using atomic force microscopy (AFM). The peak-to-valley distance (corrugation height) of latex particles in the film was monitored at different annealing temperatures as a function of annealing duration. The influence of the concentration of SAA on the rate of interdiffusion of the latex particles and hence the film morphology was investigated. The AFM results show that the kinetics obey the time/temperature superposition principles. The corrugation height of the PBMA particles in films containing SAA was found to be higher than those in the pure PBMA films. The AFM results suggest very strongly both the formation of a hard surface layer of SAA over the soft PBMA particle, and the migration of free SAA to the latex film surface during annealing. The SAA layer adsorbed on and surrounding each PBMA particle retards the interdiffusion of the PBMA molecules across the particle-particle interface and slows the gradual coalescence of the particles in film formation resulting in poorer tensile property of these films compared to that of the pure PBMA latex. The two-step kinetics of the film forming process is the direct consequence of both the interdiffusion rate of PBMA latex particle and the migration of SAA to the surface of latex films.