The potential of near-infrared (NIR) spectroscopy for monitoring the esterification of stearic acid by glycerol in a laboratory reactor was assessed. The reaction was conducted in the presence of a heterogeneous catalyst that consisted of lipase from Candida antarctica immobilized on a macroporous resin (Novozyme 435). The reaction leads to a produce a mixture of monostearate and especially distearate; no formation of tristearate occurs. The process was monitored using a fiber-optic probe to record NIR spectra for the reaction mixture at variable times. The spectra thus obtained were used to construct and validate PLS calibration models for predicting the glycerin, stearic acid, and glycerol monostearate and distearate concentrations. The ensuing method allows concentration changes in the four compounds to be monitored in real time. © 2009 American Chemical Society.