A photon-transmission method was used to probe the evolution of transparency during film formation from polystyrene (PS) particles with different molecular weights. The latex films were formed at room temperature from the PS particles having two different average molecular weights and annealed at elevated temperatures in various time intervals above the glass transition (T-g). Onset temperatures (T-H) at given times (tau(H)) for the optical clarity of films formed from low (LM) and high molecular (HM) weight PS particles were used to calculate the healing activation energies for the minor chains and found to be 22.0 +/- 0.5 and 27.0 +/- 0.6 kcal/mol, respectively. The increase in the transmitted photon intensity, I-tr, above the T-H was attributed to increase in the number of interfaces that disappeared. The Prager-Tirrell (PT) model was employed to interpret the increase in crossing density at the junction surface. The backbone activation energies (Delta E) were measured and found to be 127.8 +/- 2.5 kcal/mol for a diffusing polymer chain across the junction surface for LM and HM latex films. (C) 2000 John Wiley Ss Sons, Inc.