Dynamic light scattering and pulsed field gradient nuclear magnetic resonance were used to study ternary polymer solutions formed with poly(methyl methacrylate) and polystyrene dissolved in deuterated bromobenzene and also in deuterated toluene. The self-diffusion coeffient of each polymer and the slow mode diffusion coefficient were determined for each solution. These diffusion coefficients were then used to calculate the polymer-polymer interaction parameter chi for each solution assuming that the Borsali-Benmouna theory adequately described the dynamic light scattering observations. This procedure is complementary to that introduced by Sun and Wang. Bath procedures recognize that additional experimental information is required if the variation of chi with solution composition is to be determined. Sun and Wang measured the dynamic light scattering mode amplitudes; this work measures the polymer self-diffusion coefficients. The values of chi obtained from the bromobenzene solutions compare favorably with those obtained by Fukuda et al. using classical light scattering under optical Theta conditions. Also the variation of chi with minority polymer molar mass was similar to the predicted variation of the critical value of chi. However, the values of chi obtained from the deuterated toluene solutions were larger and did not behave in a similar manner. This anomalous behavior is explained in terms of the phase stability of the deuterated toluene solutions. However, the possibility that the Borsali-Benmouna theory is inadequate for solutions formed with a solvent of unequal affinities for the two polymers when the solutions are close to phase separation is not discounted.