The problem of obtaining the wall heat flux in the presence of unsteady heat transfer in two-dimensional, turbulent boundary layer flow is re-examined. A novel expression to produce estimates of the amplitude of the fluctuating wall heat flux has been proposed for the foregoing conditions. This expression is based on flow field measurements instead of measurements in the solid wall substrate, thus allowing us to take the flow dynamics directly into account in the analysis. The fluid effusivity, a measure of the ability of the fluid to exchange thermal energy with its surroundings, was shown to be the dominant parameter controlling the unsteady heat transfer process. (C) 2009 Elsevier Ltd. All rights reserved.