The structural rearrangement process for the HCl(H2O)(4) cluster has been studied by path-integral molecular dynamics simulations, where 'on-the-fly' calculation of the potential energy surface is done with the PM3-MAIS semiempirical level. The mechanisms of the rearrangement were analyzed using appropriate collective coordinates as well as detailed potential energy diagrams derived from low-lying stationary points. It was found that the vibrational entropy mainly determines the stability of the cluster structure especially at high temperatures. We have also found that the acidity of HCl in the cluster correlates with the coordination number of chlorine with respect water molecules. (C) 2010 Elsevier B.V. All rights reserved.