A method for the direct determination of the molecular weight of the poly(ethylene terephthalate) (PET) component in PET/acrylonitrile-butadiene-styrene (ABS) blends by gel permeation chromatography has been developed. This technique has been applied to demonstrate that an observed deterioration in the mechanical properties (e.g. Izod impact strength) of PET/ABS blends on processing can be correlated with a decrease in the molecular weight of the PET component. In the course of this work, various methods for evaluating the molecular weight and chemical microstructure of PET were examined. The use of nuclear magnetic resonance (n.m.r.) spectroscopy as a means of quantitatively identifying end groups and comonomers that are present at the 1-4% level in commercial PET (e.g. diethylene glycol, cyclohexanedimethanol and isophthalic acid) is critically evaluated with reference to the stability of PET and PET end groups in various n.m.r. solvents. New H-1 n.m.r. chemical shift assignments for cyclohexanedimethanol units are presented, and the reactivity of hydroxy end groups in trifluoroacetic acid mixtures is examined.