A study of cooling crystallisation of a potassium sulphate solution in a batch reactor is described in this paper. The effect of ultrasound on primary nucleation was investigated by measures of induction time and metastable zone width of unseeded solutions. The appearance of crystals is detected by conductivity measurements. The experimental results show that ultrasound has an effect on the primary nucleation of potassium sulphate. Ultrasound allows induction time and metastable zone width to be significantly reduced.