The present invention relates to the field of lightening keratin fibres and more particularly to the field of lightening the hair. The present invention relates to a process for lightening keratin fibres, in particular human keratin fibres such as the hair, which consists in using on the said fibres i) a substrate comprising on its surface one or more chemical oxidizing agents preferably in solid form, and ii) an aqueous composition preferably comprising one or more chemical oxidizing agents. The present invention also relates to a process for preparing the said substrate thus pretreated.