Fractions of an unsaturated polyester resin (UP), based on 1 mol maleic anhydride, 1 mol isophthalic acid, 1 mol propylene glycol, and 1 mol diethylene glycol, were obtained from an industrial reactor at various times during polyesterification. They were characterized by 1 H-NMR and chemical titration. Increasing the reaction time from 1 to 9 h led to an increase in (a) the number-average molar mass (M n ) in the range 484-1712 g/mol, (b) the maleic acid-fumaric acid isomerization from 85 to 95%, (c) the glass transition temperature ( T g ) from 251 to 267 K, and (d) the compatibility with styrene. Several of these fractions were cured with styrene, in amounts ranging from S/E ร
2-5 (molar ratio of C|C double bonds in styrene, S, and unsaturated polyester, E), using benzoyl peroxide as an initiator. Increasing M n led to an increase in the T g value and a decrease in the amount of moisture absorption. Both effects were explained by the decrease in the concentration of the polar end groups present in the UP chains. However, using a relatively high styrene fraction in the formulation (i.e., S/E ร
5) led to cured products exhibiting reasonably high T g values (approaching the pure polystyrene value) combined with a low moisture absorption. This constitutes a practical way to revalorize batches of UP resins with molar masses out of specification.