It has been shown that the strain‐energy function for an incompressible isotropic elastic material must be expressible as a function of two strain invariantsI1,I2which are elementary symmetric functions of the squares of the principal extension ratiosλ1,λ2,λ3.Valanis and Landel have proposed a strain‐energy function of the formf(λ1)+f(λ2)+f(λ3).It is shown how such a strain‐energy function can be expressed explicitly as a function ofI1andI2.Also, necessary and sufficient conditions are derived for any function ofI1andI2to be expressible in the form proposed by Valinis and Landel. It is shown how the results of problems which have been solved using the general form of the strain‐energy function can be conveniently specialized to the case when the Valanis‐Landel strain‐energy function is valid.