The enhancing effects of ultrasound on different aspects of bioprocesses have been repeatedly reported in the literature. These have usually been attributed to the mass transfer increase induced by sonication in fermentation processes. However, until now, this has not been analyzed in detail. In this work, the influence of ultrasonication on the overall gas-liquid oxygen transfer coefficient, K(L)a, has been studied as a function of reactor geometry and liquid height. Bubble columns ranging from 0.5 to 20 L were used. The effect of temperature on K(L)a was checked and no significant variation was observed between 25 and 45 degrees C. Therefore, the temperature changes caused by the application of ultrasound had no effect on the K(L)a measurement. The effects of ultrasonication were only significant when applied to small volumes resulting in power densities above 400 kW.m(-3), which are two orders higher than the standard mechanical power density supplied for mixing in fermentation (1-2 kW.m(-3)). Additionally, the enhancement obtained from ultrasound depended on the superficial gas velocity and the height/diameter ratio (HID). Sonication only enhanced K(L)a at HID ratios lower than 1, which corresponds more to the geometry of an ultrasonic bath than to a bubble column. Regardless of the ultrasound supply, when the H/D ratio increased from 0.3 to 3, the K(L)a significantly decreased regardless of the Ug value. However, for H/D ratios higher than 3, no influence of the H/D ratio on K(L)a was observed.