PURPOSE: To safely and effectively decompose dissolved hydrogen peroxide in liquid by incorporating at least one kind of hydrogen peroxide decomposition components and partially soluble barrier components into the compsn.

CONSTITUTION: When the hydrogen peroxide decomposition components(HPDC) consisting of at least one kind of the hydrogen peroxide decomposition components and barrier components are released into an aq. liquid, these components are effective for decomposing the hydrogen peroxide existing therein. The barrier components act to substantially hinder the release of the HPDC for a while after the start of the contact of the compsn. with the hydrogen peroxide-contg. aq. liquid(HPLM). The water-soluble components which dissolve into the HPLM for some time exist in the barrier components. The HPDC is so arranged that the HPDC is released into the HPLM by the dissolution of the water-soluble parts into the HPLM in association with the barrier components.