The production of chemicals from lignin by green and sustainable technologies is a promising alternative route to replace fossil fuels. Photooxidation of guaiacol, a model of lignin, to value-added carboxylic acids with microwave discharge electrodeless lamps as light source was investigated in the presence of hydrogen peroxide (H2O2). The coupled UV-vis/microwave irradiation led to a more effective degradation of guaiacol than with conventional photocatalysis. Hydroxyl radicals as active species, confirmed by electron spin resonance spin-trapping, were generated and participated in the oxidation. The effect of different operating conditions such as reaction time, guaiacol concentration, solution pH, as well as oxidant concentration, was evaluated.