Abstract: A thickened organic polymeric composition useful for molding and capable of resisting post-molding shrinkage after being cross-linked comprises a cross-linkable base resin dissolved in an unsaturated monomer, and an additive resin selected from saturated polyesters and saturated amide waxes, the additive resin being crystalline at ambient temperatures and having a melting point (T.sub.m) below a temperature (T.sub.c) at which the base resin cross-linking reaction proceeds at a significant rate. The base resin and additive resin have only a partial degree of compatibility. When cooled from a temperature between T.sub.m and T.sub.c to a temperature between T.sub.m and ambient the composition thickens, whereas, when it is heated to a temperature below T.sub.c, it reverts to a flowable composition.