A power transmission belt (10,26,32) comprising a main belt body portion of elastomeric material, a tensile reinforcement disposed in said body portion, and a sheave contact portion integral with said main body portion, said elastomeric material comprising the reaction product of: a polyisocyanate prepolymer composition prepared by reacting (i) a diisocyanate selected from the group consisting of (a) para-phenylene diisocyanate, (b) 2,6-toluene diisocyanate, and (c) a cycloaliphatic diisocyanates with trans or trans, trans geometric structure; (ii) a polyol substantially free of moieties oxidative at less than 150°C and selected from the group consisting of (a) polycarbonate polyols, (b) polyester polyols, and (c) mixtures of said polycarbonate polyols and said polyester polyols; and (iii) a triol crosslinker selected from said polyols; and adding thereto (iv) a plasticizer selected from the group consisting of C 4 -C 12 di-alkylesters of ethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glycols having a molecular weight of up to 800; and a chain extender selected from the group consisting of (a) aromatic symmetric primary diamine chain extenders, (b) mixtures of said aromatic symmetric primary diamine chain extenders and aromatic secondary diamine chain extenders, and (c) mixtures of one or more of said aromatic symmetric primary diamine chain extenders and said polyols. a polyisocyanate prepolymer composition prepared by reacting (i) a diisocyanate selected from the group consisting of (a) para-phenylene diisocyanate, (b) 2,6-toluene diisocyanate, and (c) a cycloaliphatic diisocyanates with trans or trans, trans geometric structure; (ii) a polyol substantially free of moieties oxidative at less than 150°C and selected from the group consisting of (a) polycarbonate polyols, (b) polyester polyols, and (c) mixtures of said polycarbonate polyols and said polyester polyols; and (iii) a triol crosslinker selected from said polyols; and adding thereto (iv) a plasticizer selected from the group consisting of C 4 -C 12 di-alkylesters of ethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glycols having a molecular weight of up to 800; and (i) a diisocyanate selected from the group consisting of (a) para-phenylene diisocyanate, (b) 2,6-toluene diisocyanate, and (c) a cycloaliphatic diisocyanates with trans or trans, trans geometric structure; (a) para-phenylene diisocyanate, (b) 2,6-toluene diisocyanate, and (c) a cycloaliphatic diisocyanates with trans or trans, trans geometric structure; (ii) a polyol substantially free of moieties oxidative at less than 150°C and selected from the group consisting of (a) polycarbonate polyols, (b) polyester polyols, and (c) mixtures of said polycarbonate polyols and said polyester polyols; and (a) polycarbonate polyols, (b) polyester polyols, and (c) mixtures of said polycarbonate polyols and said polyester polyols; and (iii) a triol crosslinker selected from said polyols; and adding thereto (iv) a plasticizer selected from the group consisting of C 4 -C 12 di-alkylesters of ethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glycols having a molecular weight of up to 800; and a chain extender selected from the group consisting of (a) aromatic symmetric primary diamine chain extenders, (b) mixtures of said aromatic symmetric primary diamine chain extenders and aromatic secondary diamine chain extenders, and (c) mixtures of one or more of said aromatic symmetric primary diamine chain extenders and said polyols.