Butyl acrylate methyl methacrylate, and vinyl acetate solution and emulsion terpolymerizations were conducted. Attenuated total relflection-Fourier transform infrared spectroscopy equipped with conduit and diamond-composite sensor technology was used to monitor solution terpolymerizations off-line and emulsion terpolymerizations in-line. Monomer conversion and terpolymer composition changes as a funct-ion of time were calculated by monitoring the peak height of characteristic absorbances of each monomer. Results obtained from the ReactIR (TM) 1000 reaction-analysis system agreed well with those determined by traditional gravimetry and H-1 NMR. spectroscopy. For the solution terpolymerizations? improved models developed previously to incorporate solvent effects on solution polymerizations of butyl acrylate and vinyl acetate monomers were applied to predict monomer conversion. copolymer composition, and molecular weight averages. Comparisons between experimental data and model predictions are presented.