Ultrasound irradiation and simultaneous gas bubbling into an aqueous solution of methylene blue has been studied in order to establish a cost-effective process for wastewater treatment. Various gases such as rare gases, oxygen, nitrogen, and carbon dioxide, as well as mixtures of argon and other gases have been used. The argon-oxygen mixture (70 : 30% v/v) gave the maximum degradation rate of ca. 45% after 30 min of treatment, and this was around 10% higher than that obtained with pure argon bubbling. Furthermore, the influence of the addition of a variety of metal oxide particles, including SiO(2), TiO(2), and Al(2)O(3), has been examined. The degradation rate increased by 10%, to ca. 54%, by the addition of 2 g/L alumina for a treatment time of 30 min.