A flowing afterglow system has been used to measure the gas-phase, thermal-energy rate constants for reactions between a variety of ions and water-vapour molecules at 300K. A method of measuring water-vapour flow-rates has been developed which may be readily extended to other vapours. Rate constants, together with reaction processes, are reported for the ions He⁺, Ne⁺, Ar⁺, Ar+2, N⁺, N+2, H2O⁺, C⁺, CO⁺ and O⁺ reacting with water vapour, together with measurements of the three-body rate constants for the systems O+2+ H2O + Ar and H3O⁺+ H2O + He, and the Penning ionization cross-section for the He (2³ S)+ H2O system.