Dielectric measurements were performed on poly(y-benzyl L-glutamate-co-Lleucine) (copoly(BLG,Leu)) and poly(y-methyl L-glutamate-co-L-leucine) (copoly(MLG,Leu)) in the solid state. The composition dependence of the dielectric behavior of these copolymers was examined. It was found that the introduction of a Leu residue decreased the mobility of the benzyl L-glutamate side chain of copoly(BLG,Leu), while it increased the mobility of the methyl Lglutamate side chain of copoly(MLG,Leu). The results were interpreted in terms of the side chain interaction. The hydrophobic interaction between the benzene ring of the benzyl glutamate residue and the methyl group of the leucine residue is a possible cause for the decreases in mobility of copoly(BLG,Leu). KEY WORDS Dielectric Relaxation / Poly(y-benzyl L-glutamate-co-Lleucine) / Poly(y-methyl L-glutamate-co-L-leucine) / Side-Chain Motion / Copolymer/