The solubility and nucleation of borax decahydrate in the presence of Cl- and SO42- ions have been studied by two different methods: polythermal crystallization and isothermal crystallization. The results revealed that both anions had a salt-in effect on the solubility. A noticeable widening of the metastable zone was observed after the addition of the two anion impurities. For the isothermal crystallization, two different nucleation mechanisms were obtained. When the supersaturation ratio S > 1.3, the nucleation appears to be homogeneous, whereas at the supersaturation ratio S < 1.25, heterogeneous nucleation predominates. From the prolonged induction period in the presence of the two anions, we conclude that the nucleation of borax decahydrate was suppressed after the addition of impurities. The values of the interfacial energy gamma calculated by isothermal method are lower than those obtained by polythermal method due to the underestimated values of In S.