The fracture toughness of a series of polyurethane elastomers based on polytetramethylene glycol (PTMEG) soft segment, a toluene diisocyanate (TDI) and a dimethyl-thio-toluenediamine (Ethacure E300) curative with different ratios of isocyanate to amine groups has been studied. The fracture toughness required to initiate crack growth was found to be invariant with stoichiometry but the degree of stable crack growth was found to vary with stoichiometry. This behaviour has been discussed in terms of the molecular and physical structure of the polymers, particularly the extent of cross-linking which varied with the isocyanate/amine ratio.