The development of a transient method for measuring the spectral directional emissivity of solid materials at high temperature is currently being completed by the analysis of the influence of various sources of error. A hot disk is subjected to mixed convective and radiant heat losses on one face after the rapid withdrawal of a shutter. The cooling is investigated during a short period of observation through numerical simulations of the transient temperature. The thermal problem is simulated using 3D and axisymmetric 2D codes. The progressive withdrawal of the shutter results in a delay for the transient temperature in comparison with the instantaneous withdrawal. For, an insulating material, the influence of the side wall boundary conditions remains negligible at the center of the front face. The strong influence of the convective heat transfer is demonstrated. (C) 2003 Elsevier Science Ltd. All rights reserved.