A half ester of an organic polyol made by reacting polyols with anhydrides; a thermosettable composition including (i) the half ester of an organic polyol, (ii) an anhydride, (iii) an epoxide, and (iv) a catalyst; and a thermoset product made from such thermosettable composition. The thermosettable compositions of the present invention are useful in various applications such as casting, potting, and encapsulation, such as electrical and electronics applications, and composites. The thermoset products made from the thermosettable compositions of the present invention have improved mechanical performances, especially toughness and mechanical strength, while maintaining high thermal resistance.