Three new 5-dimethylaminomethylidene-4-phenylamino-1,3-thiazol-2(5H)-ones with an hydroxyl group in the ortho, meta and para positions on the phenyl ring were synthesized in order to deduce the structural changes occurring on prototropic tautomerism of the amidine system. The existence of all the title compounds solely in the amino tautomeric form has been established in the solid and liquid (dimethyl sulfoxide solution) phases. The title compounds are analyzed from the point of view of the electronic effects and conformational freedom of their molecules. The intermolecular interactions in the crystals and their supramolecular architecture are highlighted.