A new technique based on ONION! method was implemented for simulation of liquids and solutions, in which exchange of solvents (XS) between subsystems (i.e. between QM and MM parts) is allowed. In this method, a switching layer sandwiched between 'high-level' and 'low-level' subsystems was introduced to help morphing of exchanging particle from one region to another. Test on ionic solution by molecular dynamics method shows that particle exchanges across the boundary negligibly disturb the equilibrium conditions. A rigorous treatment of the energy expression by this method also opens new opportunities for such as Monte Carlo and free energy simulations. (C) 2002 Published by Elsevier Science B.V.