Apparatus (200) for the treatment of one or more substrates comprising a treatment chamber configured to receive a liquid medium and one or more substrates; a supply of a treatment gas comprising ozone; and one or more conduits to convey said treatment gas to a bubble generator (41) wherein said bubble generator is operable to form bubbles of said treatment gas in said liquid medium, wherein the apparatus comprises a multiplicity of solid particles (46). A method of treating one or more substrates, the method comprising agitating said one or more substrates in a treatment formulation comprising a multiplicity of solid particles, a liquid medium and bubbles of ozone.