Reaction calorimetry and Raman spectroscopy were simultaneously implemented to monitor high solids content (similar to 50 wt %) semibatch emulsion polymerization reactions for two comonomer systems: vinyl acetate/butyl acrylate, VAc/BA, and butyl acrylate/methyl methacrylate, BA/MMA. Overall and instantaneous conversions and free concentration of monomer were measured by means of each technique. It was found that both techniques provided comparable results for the overall conversion, no matter whether the reactions were carried out under starved (BA/MMA) or nonstarved conditions (VAc/BA). However, instantaneous conversion and free monomer concentrations were better estimated by Raman spectroscopy when the polymerization was carried out under starved conditions. Under nonstarved conditions (higher free monomer concentration and lower instantaneous conversions), Raman spectroscopy also provided slightly better estimates.