Turbulent flow through a channel in which one wall is heated and the other is cooled is considered. Temperature profiles for Prandtl numbers equal to 0.1, 0.7, 1, 3, IO, 100, 500 and 2400 are calculated, by considering the walls to be series of sources and sinks. The spatial variation of eddy conductivity for Pr = I is interpreted in terms of the time dependency of diffusion from single sources. Support is provided for experimental studies which show that the dimensionless mass transfer coefficient varies as SC-0.704 at large Sc.