Phase diagrams (cloud points, tie lines, critical compositions) and viscosities of homogeneous solutions were determined at temperatures ranging from 20 to 100 degrees C for three representatives of the ternary system cyclohexanone/polystyrene/poly(n-butyl methacrylate) [CHO/PS/PBMA] in preparation of a study on shear influences. An only moderate increase of the two-phase region observed upon heating (LCST behavior) witnesses little heat effects upon mixing. Tie lines which are nearly parallel to the PS/PBMA edge of the Gibbs phase triangle indicate comparable solvent quality of CHO for both polymers. The quantitative mathematical description of the equilibrium behavior-required for a theoretical treatment of shear influences-was performed on the basis of the Flory-Huggins equation by means of composition and temperature dependent binary (integral) interaction parameters g(ij). The present approach avoids the use of chemical potentials and minimizes the Gibbs energy of mixing directly. Experimental data for g(CHO/PS) and g(CHO/PBMA) were available from earlier work; g(PS/PBMA) was adjusted to reproduce the measured phase diagram of the ternary system. Calculated and measured phase diagrams match very well. The rheological results were used to establish a mathematical expression describing the composition and temperature dependence of the solution viscosities; such an equation is required for the theoretical treatment of shear influences on the phase diagram.