We consider three possible mechanisms that may contribute to the positive sputtered clusters ionization: (1) ionization due to the dissipation processes of the internal vibrational energy of clusters; (2) ionization by the way of non-equilibrium electron exchange in the system "outgoing cluster-metal surface"; (3) ionization by the way of equilibrium electron exchange at the thermal equilibrium between the sputtered (evaporated) clusters and the bombarded surface. The cluster ionization probabilities, computed within the framework of these mechanisms are compared with each other and with the corresponding measurements. The comparison indicates that the mechanism of the positive ionization of Ta sputtered clusters is, primarily (1). For Ag clusters, the contributions of mechanisms (1) and (3) are comparable. In all cases under investigation, there is no noticeable contribution of the mechanism (2) to the positive cluster's ionization. (C) 2004 Published by Elsevier B.V.