The effect of solvation on the conformation of acetylene has been studied by adding one water molecule at a time. Quantum chemical calculations of the n＋ （C2H2）（H2O）n （n=1-5） clusters indicate that the H2O molecules prefer to form the OH...Tr interaction rather than the CH...O interaction. This solvation motif is different from that of neutral （C2H2）（H2O）n （n=1-4） clusters, in which the H2O molecules prefer to form the CH...O and OH...C Hbonds. For the H＋（C2H2）（H2O）n cationic clusters, the first solvation shell consists of one ring structure with two OH...Tr H-bonds and three water molecules, which is completed at n=4. Simulated infrared spectra reveal that vibrational frequencies of OH... H-bonded O-H stretching afford a sensitive probe for exploring the solvation of acetylene by protonated water molecules. Infrared spectra of the H＋ （C2H2）（H2O）n （n=1-5） clusters could be readily measured by the infrared photodissociation technique and thus provide useful information for the understanding of solvation processes.
