Dissolution of uranium materials in alkaline aqueous conditions containing H2O2 results in uranyl peroxide species in solution, including anionic uranyl peroxide cage clusters. Uranyl peroxide cage clusters are generally highly soluble in water, where they persist as aqueous macroanions. Previous studies indicate that uranyl cluster speciation and dissolution of uranium materials is impacted by the concentration of alkali metal in solution, but in these studies, high concentrations of H2O2 were used. Herein, the role of hydrogen peroxide concentration is examined relative to the dissolution of powdered UN and UO2. Lower initial H2O2 concentrations reduce dissolution of UO2 and UN and tend to produce simple (small) uranyl peroxide species rather the highly soluble uranyl peroxide clusters. H2O2 availability will have implications for uranyl speciation and solubility where spent nuclear fuel is in contact with water and where alkaline peroxide conditions are used in dissolution of nuclear material.