We establish a modification of the one-dimensional two-phase Stefan problem which is promising for the investigation of ultrafast processes in solids lasting less than the time of electron-phonon relaxation. Under such conditions the heat equations should be solved separately for ions and electrons at constant volume. We discuss the system of equations and boundary conditions for this special case and derive the computational scheme for the numerical solution. To demonstrate our new approach we calculate the electronic and ionic temperatures in an aluminum target subjected to a femtosecond laser pulse. (C) 2014 Elsevier Ltd. All rights reserved.