The change in fracture toughness of an orthophthalate based medium reactivity polyester resin with change in resin flexibility, catalyst content, cure temperature and liquid environment, has been investigated. Various specimen geometries including simple double cantilever beam and tapered double cantilever beam have been used and the results obtained with the different geometries are in good agreement. The critical flaw size for the resin has been determined and an estimate obtained of the radius of the plastic zone under plane strain conditions. The solvent absorption characteristics of the resin were found to be effected by catalyst content, resin flexibility and by residual strains. Observations of the fracture behaviour, using both fracture toughness and tensile specimens have revealed a fracture surface morphology very similar to that of glass but with limited plastic deformation occurring.