The role of solvent effects in association reactions is studied in atom-cluster collisions. Classical trajectory studies of the systems H+Cl(Ar)(n) (n = 1,12) are used to investigate the influence of size, structure, and internal energy of the "microsolvation" on the H+Cl association reaction. The following effects of solvating the chlorine in an Ar, cluster are found. (1) In the H+ClAr system there is a large "third body" effect. The single solvent atom stabilizes the newly formed HCl molecule by removing some of its excess energy. The cross section found at low energies is a substantial fraction of the gas-kinetic cross section. The molecule is produced in highly excited vibrational-rotational states. (2) Some production of long-lived HCl...Ar complexes, with lifetimes of 1 ps and larger, is found for the H+ClAr collisions. Weak coupling stemming from the geometry of the cluster is the cause for long life times. These resonance states decay into HCl+Ar. (3) At low collision energy (E = 10 kJ/mol) for H+Cl(Ar)(12), the H+Cl association shows a sharp threshold effect with cluster temperature. For temperatures T greater than or equal to 45 K the cluster is liquidlike, and the reaction probability is high. For T less than or equal to 40 K the cluster is solidlike, and there is no reactivity. This suggests the potential use of reactions as a signature for the meltinglike transition in clusters. (4) At high collision energies (E = 100 kJ/mol) H atoms can penetrate also the solidlike Cl(Ar)(12) cluster. At this energy, the solid-liquid phase change is found not to increase the reaction probability.