Colemanite is one of the most important underground riches of Turkey, having approximately 60% of the world boron deposits, and it has a large portion in the deposits. When colemanite having a 2CaO(.)3B(2)O(3)(.)5H(2)O formula is mined naturally, it contains various clay minerals. In this study, the dissolution of colemanite in aqueous oxalic acid solutions was investigated in a batch reactor employing the parameters of stirring speed, particle size, temperature and acid concentration. It was found that the dissolution rate increases with increasing temperature and decreasing particle size. The conversion rate increased until 250 mol m(-3) with increasing acid concentration and then decreased with increasing acid concentration. No important effect of stirring speed was observed. The dissolution kinetics of colemanite was examined according to heterogeneous and homogeneous reaction models, and it was found that the dissolution rate was controlled by product layer (or ash layer) diffusion process. The activation energy of the process was determined to be 9.50 kcal mol(-1). The experimental data were tested by graphical and statistical methods and it was found that the following model best fit the experimental data t = t*[1-3(1-X)(2/3) +2(1-X)]. (C) 2003 Elsevier B.V. All rights reserved.