Stability and transition problems of two dimensional boundary-layers with heated walls have been studied numerically using the linear stability theory. Incompressible stability equations have been modified to account for the variation of temperature dependent fluid properties across the layer. The equations obtained have been solved with an efficient shoot-search technique. Low speed flows of air and water have been analyzed with a wide range of heat transfer rates. In addition to the mean velocity profile characteristics, variable viscosity and density terms in the stability equations also have considerable influence on the results of the stability and transition analysis. (C) 2004 Elsevier Ltd. All rights reserved.