A fast on-line method for measuring the monomer conversion of a styrene batch polymerization reaction with near-infrared spectroscopy (NIR) has been developed. Multivariate calibration was performed, using polymer samples having temperatures around the set point of the batch reactor (75-85degreesC) and monomer conversions up to 35%. The calibration model was built in such a way that the effect of the temperature on the predicted conversion of the sample was minimized. The method was validated in a number of batch runs. In these runs, the batch temperature and molar mass distributions of the polymer were varied. At-line size-exclusion chromatography was used as a reference method for measuring the monomer conversion. Results show that on-line conversion monitoring with NIR offered overall an excellent accuracy (similar to 0.32% conversion). For high and low monomer conversions a small bias in the predicted conversion is present. The method proved to be insensitive to both relative large changes (10degreesC) of the batch temperature and to considerable changes of the molar mass distribution of the polymer.