The sonolysis of 1,4-dioxane in the absence and the presence of zero-valent iron (Fe-0) was investigated by altering bulk temperature and ultrasonic frequency conditions. In the absence of Fe-0, the sonolytic degradation efficiency of 1,4-dioxane was higher at 170 kHz as compared to that at 35 kHz. As bulk temperature increased, sononlytic degradation efficiency of 1,4-dioxane decreased at both frequencies; however, the rate decrease by the increase in temperature was smaller at 170 kHz than at 35 kHz. Adding methanol as a radical scavenger remarkably decreased the sonolytic degradation rate at 170 kHz within 60 min in all bulk temperatures. In contrast, the decrease in the degradation rate by adding methanol at 35 kHz was observed after 60 min only at 10 degrees C, but not at 20 and 30 degrees C. These results indicate that both bulk temperature and sonication frequency should be considered to optimize the sonolytic degradation. The degradation efficiency was significantly enhanced in the presence of Fe-0 especially at 170 kHz, indicating that Fe-0 can facilitate hydroxyl radical production by Fenton-type reaction mechanism during sonication,