The phase diagram of the system PBZT/polyphosphoric acid/water has been investigated using differential scanning calorimetry, optical microscopy and P-31 nuclear magnetic resonance spectroscopy. A peritectic line at higher temperatures and a eutectic line at lower temperature have been found independent from the polycondensation degree of the polyphosphoric acid (PPA). The phase transitions between the different regions in the phase diagram are strongly dependent on the condensation degree of the solvent. All phase transitions are completely reversible. The formation of crystal solvate phases is connected with the water content of the system and therefore with the polycondensation reaction of the PPA. The solvent molecules are strongly associated with the macromolecules. The nematic phase is formed by interaction of six solvent molecules per monomeric unit of the polymer and proton transfer from the solvent towards the polymer in the nematic phase.