Theoretical models have been used to investigate the effects of artificially injected H2 gas on plasma densities in teh ionospheric F region and the overlying protonosphere. Owing to large reaction rates between H2 and ionospheric O+ ions, plasma densities in both daytime and nighttime ionospheres can be greatly reduced by modest amounts of H2 gas released. 100 kg of H2 released at 300 km altitude reduces local 0+ densities by more than three orders of magnitude and produces ~5% depression in H+ densities in the overlying protonosphere. These results suggest that it should be possible to conduct controlled chemical modification experiments for investigation of many outstanding ionospheric and magnetospheric problems.