The neutral and negatively charged (H2O )(2)Ar-n, n = 1-14, clusters are investigated theoretically by use of a polarizable model potential together, in the case of the anionic clusters, with a Drude-model approach to incorporate dispersion interactions between the excess electron and the argon atoms and water molecules. The (H2O)(2)Ar-11 and (H2O)(2)Ar--(12) clusters are predicted to be "magic numbers", with their high stability being a result of their global minima adopting icosahedral-like structures. On the basis of these results, a mechanism for formation of the anions is proposed. In addition, the rapid falloff in intensity of the n greater than or equal to 7 anionic clusters in the observed mass spectrum and the absence of observable signal for the n = 10 cluster are accounted for.