Surface molecular diffusion in a latex film has been investigated by atomic force microscopy. Deformation of the particles in the plane of the film of a real latex system did not change the centre-to-centre spacing, but the peak-to-valley distance y decreased upon annealing. The theoretical and also the experimental relationship between y and time t is given by the equation y(t) = y(0) exp(-t/tau) where tau is a constant, which is related to surface tension, particle size and the surface molecular diffusion coefficient. By measuring the change of y with time, the surface molecular diffusion coefficient may be obtained. The surface molecular diffusion coefficient for a poly(methyl methacrylate-co-butyl acrylate) (50:50) copolymer latex film at 65 degreesC was found to be 0.9 x 10(13) cm(2)/s. Surface molecular diffusion in the latex film is driven by the surface tension or surface free energy.