Values of the unit thermal conductance for air flowing inside the double-skin passages must be known in order to design a heated-air, leading-edge, anti-icing system. Most of the available data on the thermal conductances for flow of air inside ducts concern the values at points far downstream from the duct entrance. (Reference 1, however, contains values of the local thermal conductance near the entrance of a circular tube.) Data which define the local heat-transfer rate throughout the length of heated straight ducts are presented herein. A steam-condensing calorimetric test section containing long, straightened ducts, typical of double-skin passages used in the leading-edges of airfoils was designed and constructed. Two types of inner skin using trapezoidal- and rectangular-shaped passages have been tested. Data required to obtain values of the unit thermal conductance were recorded as a function of distance along the passages in order to study the behavior of the air in the downstream region.