Dielectric studies on the side-chain dispersion of poly(γ-butyl glutamate)/poly(γ-benzyl glutamate) blend and copoly (γ-butyl glutamate—γ-benzyl glutamate) were performed to determine the degree of interhelix and/or intrahelix compatibility. The dielectric side-chain dispersion of the polypeptide blend demonstrated a double peak ascribed to the nearly pure components, poly(γ-butyl glutamate) and poly (γ-benzyl glutamate), indicative of the interhelix incompatibility of the butyl and benzyl groups. The results indicate nearly complete phase segregation of the two components. On the other hand, the side-chain dispersion of copoly (γ-butyl glutamate—γ-benzyl glutamate) showed a single peak in the region between those of the two homopolypeptides. The butyl and benzyl groups of this copolyglutamate were apparently compatible along the α helix axis direction, confirming the randomness of the copolymerization.