The transamidation reaction in a polyamic acid solution has been investigated using size‐exclusion chromatography and low‐angle light scattering. Mixtures of a high‐molecular weight (DP = 150) and a low‐molecular weight (DP = 10) polymer and of the high‐molecular weight polymer with monomer were studied. Mixtures were made at high and low concentrations. The polyamic acid studied is the product of the polycondensation of 3,3′,4,4′‐biphenyltetracarboxylic acid dianhydride (BPDA) with oxydianiline (ODA). In all cases the molecular weight distribution equilibrated with time to a most‐probable distribution with a DP consistent with the stoichiometry of the mixture. Equilibration required about 2 weeks for mixtures of 10% by weight at ambient temperatures. The effect of addition of a small amount (5%) of low‐molecular weight material to sample of high‐molecular weight is dramatic; for DP = 150 the molecular weight is decreased by more than one‐half. In an entangled solution or melt, this would reduce the viscosity by an order of magnitude. Copyright © 1992 John Wiley & Sons, Inc.