The effect of seeding in batch cooling crystallization is discussed on the basis of original experimental data on potassium alum-water system obtained for both controlled and natural cooling modes. Regardless of the cooling mode, unimodal product crystal size distributions of grown seeds were obtained, provided that enough seed loadings are above a critical concentration. Slow cooling was not a necessary condition to avoid secondary nucleation. The critical seed concentration can be determined easily from several batch trials with the help of a seed chart, in which mean mass size of product crystals normalized with the seed size is plotted as a function of seed concentration, with seed size as a parameter. This type of seeding effect could be observed for cooling crystallization of other systems and for crystallization employing other supersaturation-generating methods.