The molecular weight distributions of poly(ethylene terephthalate) fibers aminolyzed with selected primary amines (namely, methylamine, ethylamine,n-butylamine, and ethanolamine) were determined by using gel permeation chromatography. Initially the shape of the differential molecular weight distribution (DMWD) curve did not change although its peak shifted to lower molecular weights. As aminolysis continued, a shoulder appeared on the DMWD curve. Its appearance seemed to coincide with an increase in the density of the product. Ultimately, distinct molecular populations could be identified in the treated polymer depending on the severity of the aminolysis and the selectivity of the particular amine. The lowest is thought to represent primarily crystalline residues and has a peak molecular weight of 3400. The relations betweenMnand ultimate tenacity and breaking strain for the aminolyzed products were linear.