A direct numerical simulation of fully developed turbulent flow in a channel is used to study passive scalar transport in the immediate vicinity of a wall. The Reynolds number, based on the channel half-height and friction velocity, is 150 and the Prandtl number is varied from 1 to 10. DNS results and experimental measurements of mass transfer rates at high Schmidt numbers are used to investigate the effect of Schmidt or Prandtl number. The wavenumber spectra for temperature fluctuations show a damping of the contributions of large wavenumbers with increasing Schmidt or Prandtl number. This result suggests that the analogy between momentum and scalar transport cannot be used to define the limiting behavior of turbulent diffusivity for y --> 0. Furthermore, this limiting relation cannot be used to calculate the concentration or temperature profile since it is applicable only in the conductive sublayer, where turbulent transport is not important. (C) 2000 Elsevier Science Ltd. All rights reserved.