The entropy generation due to heat transfer and friction has been determined in transient state for laminar natural convection by solving numerically the mass, momentum and energy balance equations, using a control volume finite-element method. The variations of the total entropy generation as function of time for Rayleigh number and irreversibility distribution ratio set at 10(3) less than or equal to Ra 10(-5) and 10(- 4) less than or equal to phi less than or equal to 10(-1) and were investigated. The evolution of the maximum of entropy generation with the Rayleigh number is studied. The effect of the irreversibility distribution ratio on the maximum entropy generation and the entropy generation in steady state are analyzed. The irreversibility maps for Rayleigh number set at 10(3) less than or equal to Ra less than or equal to 10(5) and irreversibility distribution ratio phi = 10(-4) are plotted. (C) 2003 Elsevier Science Ltd. All rights reserved.