A model for suspension polymerization which relates the conversion of vinyl chloride monomer (VCM) to polymerization conditions and a tracer response measured by online gas chromatography has been developed. This model can be used to determine monomer conversion with measurements of tracer response by gas chromatograph. A series of experiments using inert mixtures of H2O/PVC/VCM/n-butane and using suspension polymerization of VCM with n-butane as tracer were carried out to evaluate the model. The solubility of n-butane in vinyl chloride was determined in the temperature range 40–70°C. The solubility of n-butane in polyvinylchloride (PVC) was estimated using nonlinear regression comparing model and experimental data for both inert mixtures and for suspension polymerization. Correlations of the solubility constants of n-butane in VCM and PVC, respectively, were obtained from the experimental data. Under the present experimental conditions, conversions can be measured online every 7–8 min using the n-butane tracer method.