The mechanical properties and fracture behaviour of polyurethanes with two hardness levels have been measured. The polyurethanes were based on polytetramethylene ether glycol (PTMEG) and toluene diisocyanate (TDI) and two different curatives, or chain extenders - methylene-bis-orthochloroaniline (MBOCA) and dimethylthiotoluene diamine (Ethacure E300). The polyurethanes made using MBOCA as the curative were found to have a lower initial modulus and less permanent set than polyurethanes of similar hardness but made using Ethacure E300. Moreover, the Ethacure E300 materials showed crack growth when the strain energy density in the sample was low, and a period of slow stable crack advance. The MBOCA cured material of lower hardness also showed slow crack growth, but crack growth started at much higher strain energy densities than for the Ethacure E300 materials. The harder MBOCA cured material did not show slow crack growth and failed rapidly. The strain energy density to cause fracture was higher than that required to initiate slow crack growth in the other materials. The reasons for the differences in the fracture behaviour are discussed in terms of the structure of the polyurethanes.