Equilibrium constants for the reactions of NH3 with the silver clusters, Ag-10 and Ag-16, have been measured in the gas phase as a function of temperature using a cluster flow reactor. The binding enthalpies, Delta H(300)degrees = -9.8 +/- 1.0 and -15.9 +/- 1.0 kcal mol(-1), and entropies, Delta S(300)degrees = -10.3 +/- 3.0 and -23.5 +/- 3.0 cal mol(-1) K-1, have been obtained for the complexes Ag10NH3 and Ag16NH3, respectively. Statistical mechanics modeling indicates that NH3 is mobile on the surface of Ag10NH3. In contrast, it is locally bound on Ag-16. The implication is that the binding mechanism involves only electrostatic interaction on Ag-10 and that Ag-16 provides a site favorable to a stronger classical chemical lone-pair bond. Thus ammonia may diffuse on Aglo and be localized on Ag-16.