Light-scattering measurements have been made on the ternary system of poly(methyl methacrylate) (PMMA) in the mixed solvent 1-chlorobutane (BuCl) + isoamyl acetate (IAA) for the two fractionated samples of the weight-average molecular weight M(w) = 1.95 x 10(6) and 9.8 x 10(4). BuCl and IAA have theta-temperatures for PMMA near 40.0 and 58.0-degrees-C, respectively. The binary mixture BuCl + IAA was found to be a nearly ideal solution by vapor pressure measurements and to be an isorefractive mixture by refractive index experiments. The mean-square radius of gyration, [s2], was determined in the mixed solvent at the condition of vanishing second virial coefficient, A2. The values of [s2] at A2 = 0 agreed with the unperturbed value obtained at the theta-temperature in the single solvent BuCl. A2 and [S2] measured at 50.0-degrees-C decreased almost linearly with increasing volume fraction of IAA in the mixed solvent. The analysis of the observed behavior of A2 by the generalized Flory-Huggins theory yielded the relation g(T) = 0.48g12, where g12 and g(T) are the binary interaction function for the mixed solvent and the ternary function for the ternary system. The relation was consistent with those obtained for cosolvent systems.