A systematic investigation of the thermodynamic and structural properties of Ar-n-HCl clusters in the n = 1-12 range by classical parallel tempering is carried out. We find considerable solid to solid mixing isomerizations that take place at temperatures well below the melting range. Unlike the related system of Ar-n-HF clusters we have studied recently, the HCl is preferentially on the surface at low temperatures (T < 5 K). However, it is thermodynamically feasible for the noble gas framework of sufficient size to cage the relatively large HCl molecule in both a highly disordered solid state and the melting anomaly region. (C) 2003 Elsevier B.V. All rights reserved.