The Taguchi method was used to determine optimum conditions for the dissolution of ulexite in water saturated with SO2 to produce monosodium pentaborate (NaB5O8) in a new process. Experiments were carried out in two stages. In the first stage of the experiments, the ranges of experimental parameters were between 14 and 50 degrees C for reaction temperature, 0.05-0.20 g mL(-1) for solid-to-liquid ratio, 5-30 min for reaction time, 200-500 rpm for stirring speed and (1440-181)mu m for particle size. The optimum conditions for these parameters were found to be 50 degrees C, 0.1 g mL(-1), 10 min, 400 rpm and 725 mu m, respectively. Under these conditions, the dissolution percentage of ulexite in water saturated with SO2 was 91.98. According to the results of this optimization, it was seen that temperature and solid-to-liquid ratio were the effective parameters. Thus, in the second stage, a new experimental design was prepared to obtain a higher dissolution percentage by using a higher solid-to-liquid ratio and a higher temperature. In this design, the optimum conditions were found as 0.25 g mL(-1) for solid-to-liquid ratio, 725 mu m for particle size, 400 rpm for stirring speed and 30 min for reaction time at 92 degrees C reaction temperature and a conversion of 100% was reached under these conditions. (c) 2005 Elsevier B.V. All rights reserved.