Abstract <p>Research works on the creation of urethane-forming compositions suitable for their use as binders in the preparation of composites by vacuum infusion are initiated. The characteristic features of the occurrence of curing processes during the urethane formation reaction which are accompanied by the appearance of a stable heterogeneous system are found. The character of interaction of the individual components of the polyurethane binder is determined by laser microinterferometry. Assumptions about the factors promoting the formation of a biphasic system are made based on the performed experiments. Ways to eliminate undesirable heterogeneity in reaction mixtures are proposed.</p>