The fundamental growth behaviors of potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP) and mixed crystals (KADP) were investigated both experimentally and theoretically. It was found that the molar concentration of ADP in the saturated solution is approximately two times that of KDP, which influences the solution stability and consequently the crystal growth rate. The intrinsic characteristics of these isomorphs allow us to explain quite well the fundamental growth behaviors. Due to the strong chemical bonding interactions between K+ and H2PO4 ions, the growth rate along the c-axis of KDP is faster than that of ADP. In addition, the competitive growth of NH4+ and K+ ions is an important factor, which results in the reduction of the growth rate along the a-axis and the split of KADP crystals along the c-axis. The current work greatly expands the understanding of the growth of KDP, ADP, and KADP. (c) 2007 Elsevier B.V. All rights reserved.