The batch crystallization of potassium chloride from aqueous solution is investigated without impurities, with zinc chloride I (30, 100, 1000 ppm), with sodium hexametaphosphate II (100 ppm) and potassium hexacyanoferrate (II) III (100, 500, 1000, 16 x 25 ppm). The dominant particle size of the pure system L(N,D) = 427 mu m increases by 40% by addition of I and by 250% by addition of II. While I does not affect, the habit II changes it from cubes to octahedrons. In cases of low concentration, III leads to dendrites. It is shown that addition of III decelerates growth and increases the metastable zone width. Hence a high supersaturation is formed, the solution impoverishes fast while III gets incorporated and therefore a strong nucleation starts in a relatively pure solution. By quasi-continuously adding 16 x 25 ppm of III the dominant particle size increases by 160% with little fines. In this case the content of III in the product is relatively low.