A particle image thermometry technique is proposed to determine the turbulent heat flux in a water channel between a sinusoidal heated bottom and a flat top wall. Reynolds numbers between 2400 and 20,500 are considered. A proper orthogonal decomposition of combined velocity and temperature fields reveals a quantitative agreement between large-scale thermal and momentum structures. The distribution of budget terms of the streamwise and wall-normal heat fluxes are similar to those for the two Reynolds stress components. Co-spectra indicate that larger scale structures make a significant contribution to the strearnwise heat flux and smaller scales are more important to the wall-normal heat flux. (c) 2006 Elsevier Ltd. All rights reserved.