In this article a ternary phase diagram for the system poly(p-phenyleneterephthalamide) (PPTA)-sulfuric acid-water is described. The concentration range between 3-23 % polymer and 92-100 % sulfuric acid were investigated. The phase system shows immiscibility of its components in the solid state, i.e. it exhibits compounds between the components and eutectic behavior. Experiments were carried out to detect the clearing temperatures of ternary PPTA-sulfuric acid-water solutions; the eutectic transition between solution, crystal solvate, and solid sulfuric acid; the transition temperatures from the solid phase into the liquid crystalline state; and the composition of the crystal solvate. Polarization microscopy, wide-angle X-ray scattering, and calorimetry were used to detect parts of the binary and ternary phase diagrams. Experiments were done to discover the influence of acid concentration on the degradation of the polymer in solution.