Literature data on the viscosity of solutions of poly(butyl methacrylate) (PBMA) and poly(methyl methacrylate) (PMMA) in diethyl phthalate (DEP) for different temperatures, including the range around and below T-g, the glass transition temperatures of the pure polymers, were evaluated by means of an approach that uses surface fractions as composition variables. The discussion of these results together with information on solutions of the isomeric polymers, poly(vinyl acetate) (PVAc) and poly(methyl acrylate) (PM(A)), in the same solvent testifies that the previously published relations remain valid for T < T-g. They enable the determination of viscosities of the pure polymers below T-g by extrapolating the solution data to vanishing solvent content. The temperature dependencies of the data obtained in this manner demonstrate that the viscosity of PBMA rises upon cooling as expected according to the WLF equation, whereas the increase is lower by several orders of magnitude for PMMA and PVAc. These observations and the temperature dependencies of the system-specific parameters of the present approach are discussed in detail.