Monitoring and controlling composition drift, is an important issue in emulsion copolymerization. Due to the heterogeneity of the polymerization system often combined with nonideal kinetics, predicting copolymer composition and changes in feed composition as a function of time is not straightforward. Therefore, accurate and fast on-line determination of partial conversions of the separate monomers is a key to understanding and controlling the copolymer system studied. For this reason on-line densimetry, resulting in overall weight conversion, is combined with on-line gas chromatography, resulting in the overall ratios of the residual monomer. Combining these two on-line data gives the partial conversion of each monomer as a function of time without the need of an internal standard. The determination of partial conversion of monomers in batch emulsion copolymerization from on-line gas chromatography and on-line densimetry is illustrated for the monomer system methyl acrylate-vinyl acetate.