The formation of HCl molecules in collisions of H atoms with Cl(Ar)₁₂ clusters is studied by classical trajectory calculations as a function of the cluster temperature. At sufficiently low impact energy (E=10 kJ/mol) a pronounced threshold effect with temperature is observed: In the solidlike regime of the cluster (T<40 K) there is a strong screening of the Cl atom by the icosahedral Ar solvation shell preventing H + Cl association. In the liquidlike phase of the aggregate (T=45 K) a sharp onset of reactivity is found with a reactive cross section of 11.4 x 10^-20 m².