Clustering of water on protonated molecular ions has been investigated by vibrational predissociation spectroscopy. Systematic measurements at different cluster sizes reveal a close resemblance of the OH stretch spectra between NH4+(H2O)(n), CH3NH3+(H2O)(n), and H3O+(H2O)(n). particularly at n greater than or equal to 6, a sharp feature, identical to that found on ice and water surfaces, emerges at 3690 cm(-1) for free-OH stretching. The feature is distinguished from the other free-OH absorption, commonly observed for small-and medium-sized (H2O)(n) clusters at 3715 cm(-1). The results, in conjunction with ab initio calculations, provide compelling evidence for 2- and 3-coordinated H2O in the protonated ion-water clusters. (C) 1997 American Institute of Physics.