Urea, thiourea or guanidine derivatives (I) are used to improve the lightening action of hair bleaches or lightening dyes. Urea, thiourea or guanidine derivatives of formula R3>R4>N-CX-NR1>R2> (I) are used to improve the lightening action of hair bleaches or lightening dyes. X : O, S or NR5> R1>-R5>H or optionally substituted aryl, 1-24C alkyl or 2-24C alkenyl. An independent claim is also included for a product for dyeing and/or lightening keratin fibers, especially human hair, comprising 0.01-10 wt.% (I).
