The possibility of reduction and stabilization of gold nanoparticles by chitosan in carbonic acid solutions under high CO2 pressure has been considered for the first time. To analyze the nature of the interaction of the system components, UV-visible and IR spectroscopies have been used. The morphology and size of chitosan-stabilized gold nanoparticles have been determined using TEM. It has been revealed that the stabilization of gold nanoparticles occurs with the participation of amino groups. It has been shown that small spherical nanoparticles are stabilized at a sufficiently high concentration of gold in the solution.