The segregation of one component of a binary, isotopic polymer mixture to a neutral surface is affected by the difference in surface energy between the two components. We begin by showing that the surface energy differences between segments of isotopic polymers at neutral surfaces, in the mean-field approximation of Scheutjens and Fleer,1 are caused by differences in their zero-point energies and are independent of the chain lengths of the two polymers. When this treatment is extended to include results obtained from lattice and off-lattice Monte Carlo simulations, however, additional terms have to be incorporated to account for the connectivity of the macromolecular chains. It is then shown that chain-length effects are particularly important in the case of a blend of hydrogenated and deuterated polymers and that the surface energy differences between the two components becomes small and approaches zero for specific values of the chain lengths of the two polymers. These results are in excellent agreement with recent neutron reflectivity data on such systems, which will be reported elsewhere. While these results restress the importance of chain connectivity, and the limitations of mean-field calculations in the context of the prediction of the thermodynamic properties of interfacial systems, they have important consequences on determining the surface segregation of a deuterated polymer from a protonated/deuterated mixture of different chain lengths. © 1991, American Chemical Society. All rights reserved.