The NMR (nuclear magnetic resonance) and GPC (gel permeation chromatography) studies of the polymerization of maleic anhydride and 1,2-propylene glycol are reported. Assignment of individual groups was made and their concentration dependence on reaction time was established. The first step of the reaction is the formation of monoesters, which, immediately after the temperature increased, reacted to diesters. The reactivity ratio between the primary and the secondary hydroxyl group of 1,2-propylene glycol was 2.6:1. The concentration of water formed was followed as a function of reaction time by the Karl-Fischer method.