Formation of crystal solvate phases was investigated in solutions of the rigid polymer poly(p-phenylene benzobisthiazole) (PBZT) in methanesulfonic acid (MSA), with particular emphasis on the role of water in inducing the phase transitions. Crystal solvates are crystalline solids which contain both polymer and solvent molecules in the unit cell. A ternary phase diagram for the PBZT/MSA/water system was determined at room temperature, and the following phases were identified : isotropic phase, nematic liquid crystalline phase, and crystal solvate forms I and II. These phases were recognized visually, as well as by light microscopy and analysis of X-ray diffraction. In the suggested model of the form I crystal solvate, four acid anions are complexed to a protonated PBZT repeat unit, with two additional acid molecules, in a unit cell. Absorption of moisture by the solution induces precipitation of the form I phase by generating an excess of acid anions beyond the solubility product. The form II crystal solvate was formed after deprotonation of the polymer, when the molar concentration of penetrating water equals the molar concentration of the free acid.