The solution copolymerization of styrene/butyl acrylate in toluene was monitored off-line using an ATR-FTIR probe. The probe was used to track the concentration of the starting materials, thereby providing conversion and polymer composition data. Off-line gravimetry and H-1 NMR spectroscopy were used as standard methods to provide a comparison to the ATR-FTIR data. A non-calibrated univariate method was first used to calculate individual monomer conversions by monitoring the peak height of characteristic IR absorbances for each monomer. An alternative, calibrated method using partial least squares (PLS) regression to relate the whole IR spectral changes to monomer concentration was also attempted. The ATR-FTIR spectroscopy with multivariate analysis showed improvement in monitoring the monomer conversion compared to the univariate method. The results agreed with those determined by traditional gravimetric and H-1 NMR. spectroscopy analysis. (C) 2003 Elsevier Ltd. All rights reserved.