Symmetric binary mixtures of partially deuterated polymers were prepared at the critical composition. The segment-segment interaction energy parameter eff was varied by adjusting the difference in deuterium content "XD between species. eff was measured as a function of temperature, "D, and molecular weight by small-angle neutron scattering. The critical point for demixing was determined to scale as eff,cN- with =1.01±0.05, where N is the degree of polymerization. This result confirms the mean-field prediction of Flory and Huggins.