The conformational transition of poly(methacrylic acid-co-styrenesulfonic acid) (PMAS) in aqueous solution was investigated by the luminescence probe method using Ru(bpy)(3)(2+) and viscometry. PMAS-8 (8 mol% styrenesulfonic acid (SS) residue) and PMAS-16 (16 mol% SS residue) revealed a unique conformational transition with changing negative charge on a polymer chain that had at least four conformations in aqueous solution. The luminescence behaviour of Ru(bpy)(3)(2+) and the viscosity characteristics of PMAS solution indicated that the conformational transition was brought about by changing the negative charge of the polymer chain.