Gas-phase ion-molecule reactions have been investigated using a pulsed-electron beam mass spectrometer. The proton affinity (PA) of tetrahydrothiophene (THT), C4H8S, has been newly determined to be 205.4 kcal mol-1 by measuring the equilibria of the proton transfer reaction between 2-methyltetrahydrofuran and THT. The thermochemical data for the hydration reaction of protonated tetrahydrothiophene, C4H8SH+(H2O)n-1 + H2O=C4H8SH+(H2O)n, with n=1-3 were measured. The small and n-insensitive enthalpy changes (-ΔHn-1, n°), 11.5, 10.4 and 9.5 kcal mol-1, with n=1, 2, and 3, respectively, have been obtained. The fall-off of -ΔHn-1, n° vs. n is small due to the positive charge dispersal in the protonated THT. The rate of proton transfer reaction from the proton-hydrates H3O+(H2O)n to THT was found to be roughly the collision rate in agreement with the findings due to Viggiano et al. [J. Chem. Phys., 88, 2469 (1988)]. Ab initio calculations have been carried out, and those energies (PA and ΔHn-1, n°) have been reproduced satisfactorily.