A composition and method oxidative colours hair based on at least one oxidative dye precursor and having effervescent properties. An aqueous oxidative hair dyeing composition resulting from mixing two aqueous compositions A and B wherein the mixed composition has a pH below or equal to 7.0, preferably is in the range of 2 to 7.0, more preferably 3 to 7. Composition A comprises at least one oxidizing agent and at least one acidic compound and has a pH between 2 and 5, and composition B comprises at least one oxidative dye precursor, optionally at least one coupling agent, optionally at least one direct dye, at least one CO2 releasing compound and has a pH between 8 and 11.
