[en] Spectroscopic studies of radical cation clusters in the gas phase provide rich information on intermolecular structures of ionized molecules and their reactions following ionization. A recent infrared spectroscopic study has revealed microscopic hydration structures of ionized water clusters. A hemibond structure of a protic molecule has been observed. Remarkable enhancement of acidities of molecules upon ionization, especially that of CH bonds, has been found. An isomerization reaction mechanism of acetone radical cation catalyzed by a single water molecule has been elucidated. (author)