The rate and extent of solution of polyvinylchloride in ester plasticizers has been measured using infrared spectroscopy. The measurement is based on shifts in the carbonyl stretching frequency of the ester linkage, similar to the familiar infrared solvent shifts commonly observed for low molecular weight materials. Double beam techniques were necessary to record the shifts. The intensity of the shifted band increases up to gelation and the rate of increase depends on the temperature of heating. The intensity of the shifted band is directly proportional to the concentration of polymer dissolved in the plasticizer and can be used to measure this concentration at any time during the gelation process. It is estimated that from 17 to 22 per cent of the polymer was dissolved at the gel point after heating at 65°C. and that about 29 per cent of the polymer was dissolved at the gel point after heating at 75°C., for the plastisols used in this study.