Borax decahydrate was directly transformed into borax pentahydrate at a temperature above the transition point. The particle size distribution of the product was evaluated using the population balance theory. Microscopic observations were coupled with the particle size distribution results to understand the mechanism of the transformation. It is observed that the transition of borax decahydrate was completed in a time scale as short as 15 minutes. The mechanism of the transformation is based on the simultaneous dissolution of borax decahydrate and crystallisation of borax pentahydrate. This mechanism results in very high supersaturation at the boundary layer of the dissolving crystals and very fine sized particles are obtained due to inevitable nucleation.