When organized phases are formed in polymer-solvent systems through first-order transition, knowledge of the temperature-concentration phase diagram brings valuable information about their nature (polymer-solvent compound, peritectic, etc.), and also about the mechanisms involved during the organization process. The construction of these phase diagrams usually obeys Gibbs phase rules, even though formation often occurs under non-equilibrium conditions. In particular, the existence of invariants (monotectic transformations, peritectic, etc.) can still be observed. Knowledge of the phase diagram is also of prime importance for understanding rheological properties, particularly for thermoreversible gels. Examples dealing with these issues are given in this paper.