The effect of a catalytic surface reaction on the convective heat transfer is studied for a cross flow to a cylinder. Platinum catalyst is coated on the surface of the cylinder which is set horizontally in a rectangular duct. The fluid is a mixture of SO2 and O-2 and the following exothermic reaction takes place on the surface of the cylinder: SO2 + 1/2O(2) = SO3 + 99 kJ/mol. The cylinder is uniformly heated electrically from the inside by the DC power supply, and the reactant gas flows through a small clearance between the cylinder and the duct wall. The heat transfer coefficient is determined based on the temperature difference between the inlet gas and the forward stagnant point of the cylinder. The heat transfer coefficient for the non-reacting gas is at least about two times higher than the conventional correlation because of the effect of natural convection and small clearance. When the reaction takes place, the heat transfer coefficient rises by 4-16%. The effect of the enhancement is proportional to the reaction rate. The same correlation of the convective heat transfer as the SO2-O-2 reaction system is observed for the reaction system: CH4 + 2O(2) = CO2 + 2H(2)O, which proceeds faster without volume change of the gas than the former reaction.