The microwave absorption properties of polystyrene (PS) copolymers containing sulfonic and methacrylic acid groups were investigated using an electron magnetic resonance spectroscopy method. It was found that, unlike PS and poly(methyl methacrylate) homopolymers, the acid groups containing PS copolymers showed three electromagnetic resonance signals at 3250, 3035, and 520 G; the first and last two signals were thought to be due to the presence of anion radicals and acid groups in PS, respectively. This result indicated that the PS copolymers exhibited magnetic field dependent low-field microwave absorption behavior. This also suggested that the low-field microwave absorption required the presence of both the aromatic and acidic groups in the materials. To explain both the low-field absorption and magnetic field dependence, we proposed a coupled mechanism including both the magnetoplasticity associated with a plastic deformation and the rotational degree of freedom of aromatic groups containing electric dipoles with natural spin defects. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3355-3359, 2008