A photon transmission method was used to probe the change with time of transparency during film formation from latex particles. Two different latex films were prepared from high (HM) and low (LM) molecular weight poly(methyl methacrylate) (PMMA) particles, which were annealed at 10 min time intervals at various temperatures above the glass transition T-g. The increase in the transmitted photon intensity I-tr is attributed to the increase in 'crossing density' at the junction surface. The back and forth activation energies (AE) were measured for HM and LM films and found to be around 29 kcal mol(-1) and 53 kcal mol(-1), respectively, for a reptating polymer chain across the junction surface. Monte Carlo simulations were performed for photon transmission through a rectangular lattice. The number of transmitted photons N-tr was calculated as a function of the mean free path of the photons. It was observed that N-tr increased similarly to I-tr as the square of the mean free path of the photons increased. (C) 1998 Society of Chemical Industry.