Abstract: Effervescent compositions are disclosed, particularly effervescent compositions useful in toilet bowls. The effervescent composition may include at least one liquid or solid hydrogen peroxide source and at least one catalyst capable of catalyzing an oxidation of the hydrogen peroxide source in the presence of a liquid, such as water. Accordingly, the effervescent composition is capable of effervescence in the presence of a liquid. The effervescent composition may also include a non-hygroscopic solvent. The presence of the non-hygroscopic solvent may assist the effervescent composition in substantially avoiding effervescence until the composition contacts the liquid. Optionally, the ratio of the hydrogen peroxide source and catalyst to the non-hygroscopic solvent is not less than 9:1.