The experiments in this study were conducted to assess the impact of impeller diameter on the metastable zone width during batch sonocrystallization of borax. A sodium tetraborate decahydrate solution saturated at 30 °C was gradually cooled at a rate of 6 °C h -1 until reaching a temperature of 15 °C. The research was carried out in a crystallizer of laboratory-scale volume of 2.65 dm 3 . Within the processing time, the mother liquor was simultaneously subjected to the action of ultrasound (A = 20 %, P = 20 %) and mechanical mixing. Mechanical mixing ensured a state of complete suspension (N = NJS). To achieve this, flat-blade turbine impellers (SBT impellers) of the following diameters were used: D 1 /d T = 0.43; D 2 /d T = 0.53; D 3 /d T = 0.63. The primary aim of this study was to investigate the influence of the SBT mixer diameter on nucleation kinetics. To achieve this, it was necessary to determine the concentration of the mother liquor using the refractometric method, as well as the absolute supersaturation during the process time under all examined process conditions. Furthermore, the influence of the impeller diameter on the metastable zone width, determined by a widely accepted visual method, was investigated. Additionally, the use of Mersmann's nucleation criterion enabled the identification of the mechanism by which the nucleation was initiated. Moreover, based on the power input into the system, the diameter of the microeddie was determined, and its impact on the width of the metastable zone was estimated. The results obtained from this investigation indicate that reducing the impeller diameter results in lower maximum supersaturation and a narrower metastable zone. Furthermore, the nucleation mechanism that initiated under all tested conditions corresponds to the primary heterogeneous mechanism. Although the results of the investigation show a reduction in the metastable zone width with decreasing microeddie diameter, the correlation between these two variables is not statistically significant, hence a larger dataset is required for more reliable conclusions.