Abstract <p>The interaction of fish gelatin and agar in the bulk of an aqueous phase with the formation of supramolecular protein–polysaccharide complexes has been studied using spectroscopy (FTIR and UV), turbidimetry, quasi-elastic laser light scattering, and scanning electron microscopy. The effects of gelatin structure (content of amino acid residues), agar/fish gelatin ratio <i>Z</i> (weight/weight), and medium (pH and ionic strength <i>I</i>) on the boundaries of the regions of stoichiometric and non-stoichiometric complex formation, the size and ζ-potential of particles and, hence, the phase state of the aqueous mixtures (colloidal solutions) of the biopolymers have been considered. Phase diagrams have been plotted for aqueous mixtures of fish gelatin and agar in the <i>Z</i>–characteristic pH and <i>I</i>–characteristic pH coordinates. The regions of different phase states of the systems have been determined, such as a single-phase solution of non-complexed biopolymers, a dispersion of fish gelatin–agar complexes, the region of the onset of the separation of coacervates from the dispersion, and the region of the complete separation of the coacervate phase and the supernatant. It has been shown that the formation of fish gelatin–agar complexes affects the microstructure of gels formed as a result of cooling aqueous mixtures of the biopolymers.</p>