A new versatile attenuated total reflectance (ATR) Fourier transform infrared (FTIR) technique has been developed for monitoring emulsion copolymerization reactions using midrange IR optical fibers. This probe was found to provide a novel opportunity for determining individual monomer conversions, as well as copolymer composition, for polymerizations carried out in aqueous media, even at a high solids content. Chalcogenide optical fibers were used to direct IR radiation from an FTIR spectrometer through an ATR probe immersed in a laboratory scale reactor and equipped with a two-reflection 45 degree ZnSe crystal. The utility of this technique for studying emulsion copolymerizations was demonstrated by monitoring a 2-ethylhexyl acrylate/styrene reaction at the azeotropic composition. Sufficient signal strength at the detector was achieved, resulting in IR spectra of high quality and resolution, while the short effective path length of the probe made it ideal for obtaining quantitative kinetic information from mid-range IR spectra in aqueous media.