Isobutylene (IB) polymerization kinetics were monitored in real time using mid-infrared FTIR-ATR spectroscopy, with a diamond-composite insertion probe and light conduit technology. Polymerization was initiated using the 5-tert-butyl-1,3-bis(2-chloro-2-propyl)benzene (t-Bu-m-DCC)2,4-dimethylpyridine (DMP)/TiCl4 (1:2:20) system in hexane/methyl chloride (60:40 v/v) cosolvents at -80 degrees C, with [t-Bu-m-DCC] = 1.90 x 10(-3) M and [IB](o) = 1.0 M. Monomer concentration as a function of time was obtained by monitoring the absorbance at 887 cm(-1) associated with the =CH2 wag of IB. The calculated apparent first-order rate constant of 8.4 x 10(-4) s(-1) was within 3% of the value determined from traditional gravimetric methods (8.6 x 10(-4) s(-1)) withing the same range of conversion. Inspection of the first-order plot generated from the FTIR data revealed a number of deviations from linearity, which were attributed primarily to a transient rise and subsequent fall in reactor temperature caused by the initial large exotherm of polymerization. An artifact associated with the method of data analysis was also identified. These small differences in instantaneous rate were not detected by the gravimetric method due to insufficient accuracy and density of data points.