Gel Permeation Chromatography GPC has become the standard technique for determining molecular size and weight distribution of soluble polymeric materials. However, when GPC is applied to mixtures containing different types of macromolecules it is difficult, if not impossible, to resolve them using conventional GPC detectors. This is true in the case of PGAnitrocellulose mixtures. Use of a Fourier Transform Infrared Spectrophotometer FT-IR as a detector for the GPC offers an approach that should enable resolution of different species by following the infrared absorption of their distinguishing functional groups. GPCFT-IR has been used to measure the molecular size distribution of nitrocellulose and to a less extent, PGA in XLDB propellant lacquers. The technique has been used to look at changes in nitrocellulose during accelerated aging of lacquer mixtures.