Cementitious waste forms are being considered for immobilizing nuclear waste before disposal. In earlier work, it was found that irradiation of a mortar formulation consisting of slag, portland cement, fly ash, water, and up to 10 wt % KCI-LiCl salt resulted in the generation of hydrogen. Yields were relatively high and the rates of generation were constant for the irradiation period investigated. The addition of small amounts of oxygenrich electron scavengers (NaN03, NaI04, KMn04, or Ag20) to the mortar was investigated as a means for reducing hydrogen yields. The addition of NaN03 reduced the hydrogen yield; changed the radiolytic products from hydrogen to a mixture of hydrogen, nitrogen, and N20; and reduced the pressurization rate after exposure to 400 Mrads. The addition of NaI04 and KMn04 reduced hydrogen yields slightly while the addition of Ag20 increased the yield. Moreover, the addition of FeS to a non-slag mortar changed the radiolysis mechanism but the addition of FeO did not. The results of these experiments provided an insight into the nature of the radiolytic reactions occurring in the mortar formulations and indicated that the radiolytic generation of gases might be controlled with the proper choice of additive.