Real-time Fourier transform near-infrared spectroscopy has been used to monitor monomer and water concentrations simultaneously during cationic vinyl ether photopolymerization. The use of near-infrared peak area methods allows the water content to be conveniently and nondestructively determined in any monomer or polymer for which the water peak has previously been calibrated by gravimetric analysis. Although the shape of the absorption band due to absorbed water in a monomer changes with the quantity of water, the integrated intensity from about 5350 to 4900 cm(-1) can be correlated directly to the water concentration, and this region is well removed from the vinyl-based absorption at approximately 6190 cm(-1). This approach provides a highly informative, dynamic technique for examining the influence of moisture on polymerization reactions. Significant differences have been observed in the effects of absorbed water on the cationic photopolymerization kinetics of vinyl ether monomers with or without an -OH group. Along with the rapid consumption of water coupled to vinyl ether polymerization, acid-catalyzed hydrolysis reactions have also been spectroscopically observed, giving rise to the formation of aldehyde groups. (C) 2004 Wiley Periodicals,Inc.