In a method and apparatus for cleaning or processing a workpiece, a process gas is brought into contact with the workpiece by diffusion through a heated liquid layer on the workpiece, and by bulk transport achieved by entraining the gas in a liquid stream, spray or jet impinging on the workpiece. The process gas, which may be ozone, is entrained in the liquid via entrainment nozzles. Use of entrainment and diffusion together increases the amount of gas available for reaction at the workpiece surface, increases the reaction rate, and decreases required process times.