An agent for lightening keratin fibers, which, with regard to its weight, includes 20 to 75 wt. % of oil(s), 0.05 to 5 wt. % of polymer(s) from the group of copolymers of ethylene/propylene/styrene and/or copolymers of butylene/ethylene/styrene, and/or copolymers of butylene/propylene/styrene, and 1 to 70 wt. % of peroxydisulfate(s), wherein the fibres contain 1 to 44 wt. % of potassium peroxydisulfate and 0 to <5 wt. % of sodium peroxydisulfate, and 0 to <5 wt. % of ammonium peroxydisulfate. The agent has increased storage stability, whereby not only the physical stability (settling, phase separation) but also the chemical stability (decomposition of the peroxide salts) is improved.
