C. C. Walker first demonstrated that size exclusion chromatography (SEC) is an effective analytical technique for observing molecular weight distribution changes (MWD) of polyamic-acids with time. The specific polyamic-acid studied was that based on pyromellitic dianhydride and 4,4′- oxydianiline (PMDA/ODA), and the MWD narrowed on aging. Essentially, a decrease occurred in the weight average molecular weight (Mw), but no change took place in the number average molecular weight (Mn). Redistribution of molecular weight was postulated to be due to an equilibration reaction arising from the reversibility of amic-acids. Additional data on Mw decreases with time of polyamicacids other than PMDA/ODA have been reported by Miwa and Numata and Kreuz, and although trends are similar, rates of equilibrations of various polyamic-acids appeared to be partially due to backbone structure. The dianhydrides of 3,3′,4,4′-biphenyl dianhydride (BPDA) and 3,3′,4,4′- oxydiphthalic anhydride (ODPA) tended to give more stable polyamic-acid solutions.