The effect of ultrasound on reactive crystallization is studied by measuring the induction time when nucleation is predominantly homogeneous. Barium sulphate is used as the working substance precipitated by mixing aqueous BaCl2 and Na2SO4 solutions. The experiments were carried with various energy inputs. It is observed that at a given supersaturation level, the induction time significantly decreases with increasing ultrasonic energy. The mechanism of the ultrasonic effect on homogeneous nucleation is analyzed. The main effect of ultrasound is to increase the diffusion coefficient (D-AB). The surface tension (gamma(CL)) changes only slightly or remains constant in the ultrasonic field. This suggests that diffusion acceleration is the main reason for the reduction of the induction time. The relationship between DAB and energy input is also investigated and it is found that D-AB-D-AB0 has an exponential relationship with energy input. (c) 2005 Elsevier Ltd. All rights reserved.