The most important problem in boric acid production from colemanite ores with H2SO4 is the formation of MgSO4 impurity due to the partial decomposition of clay minerals in the reaction media. Increase of MgSO4 concentration in solution may be balanced by the discharge of mother liquor which leads to decrease the efficiency of the process. Therefore, the intake rate of MgSO4 should be lowered for obtaining high purity product in a high yield process. In order to control the intake rate of MgSO4 impurity, propionic acid, which does not decompose the clay minerals, is used in an acid mixture with H2SO4. Batch wise laboratory experiments showed that the higher the propionic acid in reacting acid mixture, the lower the magnesium intake rate. When 10% of required H2SO4 replaced by propionic acid, magnesium intake concentration decreased to approximately half of the value obtained in the reaction with H2SO4 (C) 2008 Elsevier B.V. All rights reserved.