The most important problem in boric acid production from colemanite ores with H(2)SO(4) is the formation of MgSO(4) impurity due to the partial decomposition of clay minerals in the reaction media. Increase of MgSO(4) 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 MgSO(4) should be lowered for obtaining high purity product in a high yield process. In order to control the intake rate of MgSO(4) impurity, propionic acid, which does not decompose the clay minerals, is used in an acid mixture with H(2)SO(4). 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 H(2)SO(4) replaced by propionic acid, magnesium intake concentration decreased to approximately half of the value obtained in the reaction with H(2)SO(4) (C) 2008 Elsevier B.V. All rights reserved.