Sodium acid phthalate (NaAP), a new semi-organic nonlinear optical material, has been synthesized and single crystals were grown from aqueous solution. The FTIR spectrum confirms the compound formation. Single crystals of NaAP have been grown by slow evaporation of solvent at room temperature up to dimensions of 10 min x 5 min x 3 mm. Powder X-ray diffraction study on grown crystals shows that they belong to an orthorhombic system. The UV-Vis-NIR transmission spectrum has been recorded in the range 200-1100 nm. The second harmonic generation conversion efficiency of NaAP was determined using Kurtz powder technique; it was observed that it has double the efficiency of KAP crystals. The laser damage threshold value was determined using a Q-switched Nd:YAG laser operating at 1064 nm and with 65 ns pulses in single shot mode. (c) 2005 Elsevier B.V. All rights reserved.