A study has been made of an unsaturated polyester resin prepared from maleic anhydride, phthalic anhydride and 1,2-propene glycol. The resin is transformed by alkalization with ammonia into a water-soluble emulsion, which was polymerized with initiators and accelerators. It was found that water acts as an inert diluent in the emulsion. By measuring the temperature of the reaction mass during polymerization, the actual conversion has been estimated. The latter is lower than literature values. A correction of the method of A. Rojas et at for the determination of the actual conversion is proposed, and the polymerization heat is thus determined. A value of 478 J/g is obtained, which is higher than reported. The apparent order of the polymerization reaction has also been determined; it changes from 5 to 18 with reaction progress.