Precipitation of potassium dihydrogen phosphate from aqueous solutions is studied in presence of aluminium ions. Conductivity experiments, chemical analyses and microscopic observations are performed. They allow to determine the kinetics of precipitation, and to know the effect of the additive on the separate steps of the crystallization : primary nucleation, growth, and secondary nucleation. Morphological aspects are particularly studied. Some qualitative and quantitative interpretations of the effects of aluminium ions are proposed.