The present invention relates to the use of one or more particular dihydroisoquinolinium salts of formula (I) for treating keratin materials, in particular keratin fibres, especially human keratin fibres such as the hair. The invention also relates to a process for treating keratin materials using said salts and optionally one or more chemical oxidizing agents. A subject of the invention is also a composition for lightening keratin materials, comprising one or more dihydroisoquinolinium salts and one or more chemical oxidizing agents. The present invention also relates to one or more particular dihydroisoquinolinium salts and also to compositions containing them, in particular compositions comprising a physiologically acceptable medium.