Ultrasound is commonly used to influence crystallization processes to get narrow crystal-size distributions and uniform products. But the mechanism, by which ultrasonic irradiation induces nucleation is not yet completely understood. It is still unclear whether the cavitation bubble surface itself acts as nucleation center or the resulting high pressures and temperatures during the collapse have any influence. In this work, we replaced the cavitation bubbles by gas bubbles to check if the bubble surface itself acts as nucleation center. The difference between these bubbles is that the former ones are firstly expanding and then collapsing and the latter ones are just expanding so that the supposed mechanism could be separated. We investigated the effect on the crystallization behavior of dodecanedioic acid in several solvents (propyl acetate, ethyl acetate and acetonitrile) in terms of inducing nucleation and influencing both, the crystal-size distribution and the supersaturation profile. Therefore, the initial supersaturation ratio, the gassing period, the bubble content as well as the specific bubble surface were varied. The experimental results show that gassing can reduce the metastable-zone width and has influence on the crystal-size distribution. It is assumed that nucleation takes place due to the bubble surfaces provided, acting like foreign particles, so that the nucleation mechanism seems to be a heterogeneous one. (C) 2009 Elsevier Ltd. All rights reserved.