The present invention relates to an aqueous composition for treating hair especially for lightening hair. The aim of the present invention is to provide solution to the above mentioned problem, and especially aims at providing an aqueous composition for effectively lightening color of natural or artificially colored hair, especially oxidatively colored hair. Further aim of the present invention is to provide aqueous coloring composition for homogenously coloring hair. It has been found out that an aqueous composition comprising an alkalizing agent, a guanidine salt and another salt selected from carbonates, bicarbonates, phosphate and hydrogenphosphates with sodium or potassium effectively lightens color of natural or artificially colored hair in the presence of an oxidizing agent and provides homogeneous coloration from root to tips with hair dyes especially with oxidative hair dyes.
