The steady-state fluorescence technique was used to examine the healing and interdiffusion of polymer molecules as a function of solid content during annealing of latex films above the glass transition (T-g). Films were prepared from a mixture of naphthalene (N)- and pyrene (P)-labeled poly (methy methacrylate) (PMMA) latex particles. Above T-g, interdiffusion of polymer chains was observed by detecting the steady-state energy transfer from excited naphthalene to pyrene molecules. Various latex films with different latex content were used to measure the critical occupation percent for the reliable steady-state fluorescence measurements. Diffusion activation energies in these latex films were measured and found to be around 30 kcal/mol, which was attributed to the backbone motion of PMMA chains.