Heat transfer to supercritical H2O/CO2 mixtures (24 MPa, 310 to 430 °C, and CO2 mass fractions up to 18.5%), the working fluids of a novel power generation system with coal gasified in supercritical water, was experimentally investigated for typical working conditions of this system. For these conditions, i.e., high mass velocities (above 1200 kg m−2 s−1) and low heat flux (below 300 kW m−2), the convection heat transfer coefficients (HTCs) of supercritical pure fluids usually increase with temperature, peak near the pseudo-critical point, i.e., heat transfer enhancement, and then decrease for higher temperatures. Here, we experimentally demonstrated a new heat transfer enhancement phenomenon for supercritical H2O/CO2 mixtures. A high-temperature and high-pressure apparatus was setup to measure the convection HTCs of the supercritical H2O/CO2 mixtures. Experimental results show that surprisingly two distinct peaks of convection HTCs appear, with one corresponding temperature being the pseudo-critical point of the H2O/CO2 mixture, i.e., the thermophysical property variation induced mechanism, and the other one being the critical miscible point of the mixture, i.e., the dissolution-induced mechanism. These results pave the way to efficient heat transfer devices that use supercritical mixtures as heat transfer fluids.