Vibrational predissociation spectra are reported for size-selected NH4+∙(H2O)n clusters (n = 5–22) in the 2500–3900 cm−1 region. We concentrate on the sharp free OH stretching bands to deduce the local H-bonding configurations of water molecules on the cluster surface. As in the spectra of the protonated water clusters, the free OH bands in NH4+∙(H2O)n evolve from a quartet at small sizes (n<7), to a doublet around n = 9, and then to a single peak at the n = 20 magic number cluster, before the doublet re-emerges at larger sizes. This spectral simplification at the magic number cluster mirrors that found earlier in the H+∙(H2O)n clusters. We characterize the likely structures at play for the n = 19 and 20 clusters with electronic structure calculations. The most stable form of the n = 20 cluster is predicted to have a surface-solvated NH4+ ion that lies considerably lower in energy than isomers with the NH4+ in the interior.