Reactions of borax with organic acids were studied in a batch mode. Product boric acid resulting from the reaction of solid borax with liquid propionic acid was found to deposit on the surface of the unreacted borax core to form an ash layer. Sharp interface unreacted core model was used for simulation of this process. The analytical solution to the model involving diffusion through a liquid boundary film, diffusion through the solid reaction products, and surface reaction was obtained. Reaction of borax solution with oxalic acid crystals exhibited a different route, i.e., reactive precipitation process. Crystallization of boric acid took place in the solution phase after complete dissolution of oxalic acid crystals. Nucleation and growth rates were evaluated from a series of experiments by the method of s plane analysis. Relative nucleation kinetics were correlated in terms of the significant process variables.