Infrared spectroscopy of large-sized protonated methanol clusters, H+(MeOH)(n) (n = 4-15), was carried out in the OH stretch region to characterize the development of the hydrogen bond network with the cluster size, n. The band intensity of the free OH stretching mode decreased with it, and the band finally disappeared at n = 7. On the other hand, the broad absorption band due to hydrogen-bonded OH stretches exhibited a remarkable shift with the cluster size, and it finally converged on 3300 cm(-1) for n greater than or equal to similar to10. The size dependence of the infrared spectra was morphologically interpreted in terms of the formation of the bicyclic hydrogenbonded structure of the clusters.