We clarified the birefringence properties of poly(methyl methacrylate), poly(ethyl methacrylate), poly(isobutyl methacrylate), poly(cyclohexyl methacrylate), poly(isopropyl methacrylate), and poly(tert-butyl methacrylate). We demonstrated that the conformational change in polymer molecules that causes orientational birefringence differs from that causing photoelastic birefringence. Orientational birefringence depends mainly on the orientation of the main chains of the methacrylate polymers above T-g. On the other hand, photoelastic birefringence in elastic deformation below Tg depends mainly on the orientation of the side chains while the main chains are scarcely oriented. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2029-2037, 2010