A nuclear magnetic resonance (NMR) method has been developed to monitor on-line lipase-catalyzed esterification reactions without the need to sample the reaction medium, The technique, through H-1 NMR, measures the concentrations of alcohol, ester, hydroxylic hydrogens in the organic phase, and hydroxylic hydrogens in the aqueous phase, if any. Also, the chemical shift evolution of the two types of hydroxylic hydrogens has been followed, providing information on water content of the organic phase and on the appearance of a distinct aqueous phase. As Far as C-13 NMR is concerned, it has been possible to measure, first the acid and the ester concentrations in ?he carbonyl region, and second, the alcohol and the ester concentrations in the methylene region. All H-1 and C-13 results are in agreement with one another. Furthermore, NMR allows for the choice of detection zone. Preliminary studies on the solid phase proved the presence of much more water in the solid phase than in the organic phase, and also gave evidence of the existence of two types of esters, one in the organic phase, mainly associated with the acid, and the other one not associated with the acid, most probably entrapped within the solid enzyme.