In order to evaluate the stored energy functionWfor elastomeric materials, at least biaxial experiments must be made. Two of the biaxial testers developed at the Jet Propulsion Laboratory are an inflated cylinder device and a second biaxial sheet tester. Some preliminary experimental results obtained with these two devices at small values of strain invariants are presented for a polyurethane elastomer. These results are analyzed without making the assumption that the elastomer is incompressible, a fact which modifies the analysis of multiaxial or large‐strain behavior. The results from the biaxial sheet tester are consistent with those obtained from an inflated cylinder of the same material, though experimental difficulties prevent a detailed comparison. Nevertheless, both series of experiments show that all three terms,∂W/∂Ii,whereIiare the three strain invariants, depend strongly on the invariants. Moreover,∂W/∂I2is negative, in agreement with theoretical expectations. Thus, the form ofWin the region whereIi<4is very complex.