Abstract <p>The paper is devoted to elucidation of characteristic features of the heat transfer through a solid wall into a liquid heat-transfer agent in conditions of a confined space and a high heat flux density typical of modern mini-size systems. The objects of investigation were polypropylenglycol (PPG-425 and PPG-725) aqueous solutions with the lower critical solution temperature (LCST), transiently superheated with respect to the liquid—liquid equilibrium line and the liquid—liquid spinodal. Superheat was performed by the method of controlled pulse heat generation in a micro-size wire probe. The heating duration was from 20 to 100 ms, and the probe temperature varied from 373 to 773 К at a supercritical pressure. The attention is focused on a search for the explanation of the considerable changes in heat-transfer intensity accompanying the decomposition of an unstable solution, as functions of the changes of the water content in the initial solution. A physical model of the spinodal decomposition of aqueous solution of PPG has been suggested. According to this model, the effect of the asymmetry of heat-transfer intensity with respect to the critical concentration of the solution is related to the concentration dependence of the solution viscosity. The results will serve as a basis for choosing the optimum composition for transferring high-density heat fluxes in solutions with the LCST.</p>