Two different sets of powder latex film were prepared from high-and low-molecular-weight (HM and LM) poly(methyl methacrylate) (PMMA). The optical transmission method was used to the study film formation induced by organic vapor. Various HM and LM films with the same latex content were prepared separately from the PMMA particles and exposed to vapor of a chloroform-heptane mixture in various volume percents of chloroform. Transmitted light intensities, I-tr, from these films increased in time under vapor exposure, which was attributed to the increase in "crossing density" at the junction surface. The Prager-Tirrell model was employed to obtain the back-and-forth frequency, v, of the reptating polymer chain during film formation induced by solvent vapor. v values were obtained and found to be strongly correlated with the percent of chloroform in the solvent mixture and the molecular weight of the polymer chain. It is observed that high-molecular-weight chains reptate much more slowerly than low-molecular-weight chains.