Abstract: The model used as the basis for the calculation of nucleation Kinetics from measurements of the widths of the metastable zone was refined. Metastable zone widths, represented by the critical under-cooling at which crystalline particles being first detected, were determined for the aqueous solution of sodium tetraborate decahydrate (borax) by using an apparatus equipped with computer and laser measuring device. The effects of trace amounts of n-pentane in the solution on the solubility and metastable zone width were investigated. Nucleation parameters were obtained and discussed in terms of the refined model.