Abstract Photolytic degradation of acid orange 7 (AO7) under microwave irradiation was investigated in which a novel microwave electrodeless lamp (MWL) was employed as light source. The novel MWL brings a new way to study the simultaneous effect of both UV–vis light and microwave irradiation on degradation of dye pollutants. Color, TOC, pH and inorganic ions concentrations with reaction time were monitored to evaluate the ability of MWL to degrade AO7. It was found that the coupled UV–vis/microwave irradiation led to excellent performance on decolorization and mineralization. AO7 solution could be completely decolorized at 80 min and mineralized at 150 min. Inorganic ions produced in the solution proved that AO7 was completely destroyed. The mechanism of MWL photolytic degradation of AO7 was investigated by examining the presence and role of some important oxidative species, such as superoxide (O 2 −), hydroxyl radical (HO ), H 2O 2 and O 3, using appropriate quenchers. The results showed that a large amount of H 2O 2 and O 3 could be generated in AO7 solution by MWL irradiation and the degradation of AO7 was mainly achieved by the attack of HO radicals resulted from photolysis of H 2O 2 and O 3. Direct oxidation of AO7 by O 3 and O 2 − also played a few roles in degradation of AO7.