Changes in the mobility of bipolar ions due to water vapor in the medium gas and the effect of the change on the aerosol particle charging states were investigated, both experimentally and theoretically. Particle charging equations regarding the mobility distributions of bipolar ions were also derived from basic particle charging equations (Marlow and Brock, 1975; Adachi et al., 1985; Reischl et al., 1996) and confirmed by comparison with experimentally obtained data. For positive ions, the peak electrical mobility was very stable and independent of water vapor concentration, but for the case of negative ions, a considerable increase with water vapor concentration was found. The change in negative ion mobility had a more significant effect on the positive charging ratio of aerosol particles than a negative one, indicating that negatively charged particles are more amenable than positively charged ones to aerosol particle measurement using a differential mobility analyzer. Since a better agreement was found between experimental and calculated results by the solution for the ion mobility distribution than the basic charging equation, it would be expected that particle charging equations regarding mobility distribution would be applicable to a determination of particle charging ratio when many types of bipolar ions are simultaneously present in a particle charging space.