Measurements of heat-transfer coefficients in the critical region of ethane-n-heptane mixtures are reported for a vertical plate. The effect of retrograde phenomena on convective heat transfer is investigated. The heat-transfer coefficients increase strongly if retrograde condensation takes place in the boundary layer. Due to retrograde condensation, a local well defined double phase change - condensation and evaporation - is induced inside the boundary layer without changing the macroscopic behavior of the convective flow.