The copolymerization of styrene/butyl acrylate in a miniemulsion was monitored inline with an attenuated total reflectance/Fourier transform infrared (ATR-FTIR) probe. ATR-FTIR spectroscopy was used to track the concentration of the monomers, thereby providing conversion and polymer composition data. Offline gravimetry and H-1-NMR spectroscopy were used to provide a comparison with the ATR-FTIR data. Because of inconsistent results with a univariate method, a multivariate or partial least squares calibration method using the full spectra of the reactions was selected and gave excellent results. No statistically significant differences were found between the offline and ATR-FTIR spectroscopy data coupled with multivariate statistics, and this confirmed that ATR-FTIR spectroscopy is a reliable tool for monitoring the conversion and polymer composition in miniemulsion polymerizations. (C) 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 46-52, 2007