Various applications of ultrasound in heterogeneous reactions have been published.2 This paper describes the direct formation of azides from activated primary organic halides and aqueous sodium azide solution under ultrasonic irradiation D»]. A mixture of the alkyl halide and saturated aqueous sodium azide is irradiated in an Erlenmeyer flask using a commercial ultrasonic cleaner. After 1-4 h, the corresponding alkyl azide can be easily condensed or extracted from the water phase. The advantage of the new method is the direct use of sodium azide as a cheap, available azide ion source. Additional reaction steps to generate another azide group-transfer reagent like hydrazoic acid, trimethjlsilyl azide 4 or tetramethylguanidinium azide are not necessary. The use of co-solvents 6 like ethanol often leads to seperation problems, especially from low boiling organic azides as in this case. Water as sole solvent can easily be seperated by decantation and usual drying operations. The use of phase-transfer catalyst 7 is not necessary, because ultrasound works as an effective "non-chemical phase-transfer reagent". If the alkyl halide and the saturated aqueous sodium azide solution are left 1-4 h at 60 °C without ultrasonic irradiation, nearly no conversion was observed. Ultrasonic irradiation is decisive for good conversion in this liquid