Effects of the addition of L-lysine monohydrochloride dihydrate (L-LMHCl dihydrate) on the growth and various properties of ammonium dihydrogen orthophosphate (ADP) single-crystal grown by the slow cooling method have been studied. The grown crystals were subjected to XRD, FT-IR, UV-vis, TG/DTA and dielectric studies. The cell parameters and the crystallinity of the grown crystals were estimated by the single-crystal and powder X-ray diffraction analysis, respectively. The grown crystal has good transparency in the entire visible region, which is an essential requirement for a nonlinear crystal. Dielectric constant and dielectric loss were measured for the grown crystal for different frequencies and temperatures. (C) 2008 Elsevier B.V. All rights reserved.