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 (T-g) 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 T-g 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. 0 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.