The transesterification of molten ethylene-vinyl acetate (EVA) copolymers by octanol with sodium methoxide as catalyst in an extruder has been monitored by online near IR (NIR) spectroscopy. A total of 60 NIR spectra were acquired for 37 min with the last spectrum recorded 31 min after the addn. of octanol and catalyst was stopped. The exptl. spectra show strong baseline fluctuations which are cor. for by multiplicative scatter correction (MSC). The chemometric methods of the orthogonal projection approach (OPA) and multivariate curve resoln. (MCR) were used to resolve the spectra and to derive concn. profiles of the species. The detailed anal. reveals the absence of completely pure variables which leads to small errors in the calcn. of pure spectra. The initial estn. of a concn. that is necessary as an input parameter for MCR also presents a non-trivial task. We obtained results that were not ideal but applicable for practical concn. control. They enable a fast monitoring of the process in real-time and resolve the spectra of the EVA copolymer and the ethylene-vinyl alc. (EVAL) copolymer to be very close to the ref. spectra. The chemometric methods used and the decompd. spectra are discussed in detail.