The main elementary processes were identified during simultaneous crystallization of NaCl and KCl from aqueous solutions by the batchwise evaporative method using NaCl seeds. In the early stages of the batch operation, when only NaCl crystallizes, the main elementary processes are molecular crystal growth, agglomeration, and secondary nucleation. When the eutonic condition is exceeded, a KCl primary nucleation event takes place in solution. Subsequently, part of the resulting KCl particles agglomerate with the NaCl particles. Besides, epitaxial growth of KCI upon NaCl crystals takes place. Consequently, the product is comprised of mixed composition polycrystalline particles and single crystals of each salt. The elementary phenomena just described were sensitive to crystallization conditions such as evaporation rate, seed size, and seed content, suggesting the possibility of controlling the morphological features of the particulate product as well as the chemical composition of its size fractions. Such knowledge is important in the context of wastewater treatment, where the particulate product should be suitable either for disposal as a residue or for further processing in easily separable size fractions for exploitation of valuable components.