The melt-extrusion transesterification of ethylene-vinyl acetate (EVA) copolymer to ethylene-vinyl alc. (EVAL) copolymers was monitored by online near IR (NIR) spectroscopy. A total of 60 NIR spectra were measured within 37 min after the initial addn. of octanol (reagent) and catalyst (sodium methoxide) at the exit of the extruder by use of a fiber-optic probe. The most significant intensity change is obsd. for a band at 7089 cm-1 due to the first overtone of an OH stretching mode of the EVAL copolymers. We can monitor the progress of the reaction by plotting the peak intensity at 7089 cm-1 only. A principal component anal. (PCA) was carried out for the series of NIR spectra in the 7300-6900 cm-1 region. A score plot of PCA factor 1 is almost identical with the plot of the peak intensity at 7089 cm-1. Calibration models for predicting the vinyl acetate content in EVA copolymers have been developed by use of partial least squares (PLS) regression. The correlation coeff. and std. error of prediction are 0.96 and 0.85%, resp., indicating that the described technique can be used to monitor the transesterification reaction.