Vibrational predissociation spectroscopy of protonated methanol clusters (tetramers and pentamers) reveals linear and cyclic structural isomers in a supersonic expansion. The cyclic pentamer, containing a five-membered ring, is identified by its characteristic free-OH stretch at 3647 cm(-1) and hydrogen-bonded OH stretches at 3448 and 3461 cm(-1). Ab initio calculations indicate that the excess proton in these clusters can be either localized on one methanol unit in cyclic CH3OH2+(CH3OH)(3) and linear CH3OH2+(CH3OH)(4) or delocalized between two methanol molecules in linear C2H9O2+(CH3OH)(2) and cyclic C2H9O2+(CH3OH)(3). Dynamic intracluster proton transfer can occur upon repeated ring opening and closing. The association of this process with the anomalously high proton mobility in liquid methanol is discussed.