Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was used to study the mechanism for consumption of isocyanate groups from a dimethyl meta-isopropenyl benzyl isocyanate (TMI(R)) monomer. A series of polystyrene/poly(butyl acrylate-co-TMI) [PS/P(BA-TMI)] core/shell structured latexes with different core/shell ratios were prepared by seeded emulsion polymerization techniques. These latexes were air-dried on ATR-FTIR germanium (Ge) discs while the FTIR spectra were taken during the film-formation process. The amounts of isocyanate moieties which were reacted during polymerization and the film-formation process were determined quantitatively using the FTIR data combined with titration calibration of the isocyanate groups. The thickness of the shell in which the TMI monomer was incorporated was found to be very important in determining the extent of consumption (hydrolysis) of the isocyanate groups during either polymerization or film formation.