The thermal, rheological, and mechanical behavior of copolymers of lactide, isosorbide, and three different phthalic acids are studied in a wide range of compositions. A linear behavior in glass transition temperature (Tg) with respect to the mole fraction of isosorbide and phthalate content in the copolymers is observed. This behavior is in quantitative agreement with the well-known Di Marzio-Gibbs equation, which correlates the Tg with the mole fraction of flexible bonds in the monomers. The viscoelastic properties are found to obey the Williams-Landel-Ferry (WLF) behavior. This allows us to compare the zero-shear viscosity η0 of all copolymers with different molecular weights at a reference temperature, thus studying the influence of the different phthalic acids on the viscosity. Again, a linear mixing rule on the basis of mole fractions is established in logarithmic scale of viscosities.