For some pure elastomers, the strain‐energy function can be expressed as the sum of three identical functions of the principal stretch ratios, that is the strain energy is of the Valanis‐Landel form. Methods of measuring this function are derived and demonstrated. Examples are given of evaluation from extension and compression (biaxial extension) and from torsion of a cylinder. The character of the strain‐energy function about the undeformed state(λ=1)is examined in detail.