We self-consistently solve models for the plasma transport and chemistry and the neutral density. These models yield a plasma composition in Saturn's magnetosphere which is consistent with the high neutral densities known to be present. We find that the molecular ions OH+ and H2O+ make up over 80% of the ion density, with more H2O+ than OH+ inside 6 RS and equal amounts of OH+ and H2O+ outside 6 RS. These predictions will be directly tested by Cassini in the near future.