Face growth rates were measured on (101) and (100) faces of KDP (Potassium Dihydrogen Phosphate) and ADP (Ammonium Dihydrogen Phosphate) crystals at different supersaturations in the presence of aluminium, ferric and chromium ions added as habit modifiers. The growth rate of the (100) face decreases dramatically. The relative habit-modifying powers of the trivalent cations (Al3+ < Fe3+ < Cr3+) are confirmed. When ADP crystal grows, adsorption on the ledge is important for the reduction of face growth rate by aluminium and ferric ions. Adsorption at the step is important for chromium ion.