Measurements are reported on the relationship between the principal stresses t₁ and t₂ and the corresponding extension ratios lambda ₁ and lambda ₂ in the general biaxial strain of a rubber sheet. The resulting data are shown to be consistent with the Valanis-Landel hypothesis, according to which the strain energy W is a separable function w( lambda ) of the principal extension ratios. The formulation of the series for lambda (dw/d lambda ) in terms of strain invariants I₁ and I₂ enables delta W/ delta I₁ and delta W/ delta I₂ to be accurately calculated for any values of I₁ and I₂. It is concluded that both delta W/ delta I₁ and delta W/ delta I₂ are functions of both I₁ and I₂. The results confirm the conclusions of earlier workers that the empirical Mooney constants derived from uniaxial extension or compression bear no direct relation to the values of delta W/ delta I₁ and delta W/ delta I₂.