Potassium sodium phthalate monohydrate (K0.78Na1.22[C6H4(COO)(2)]center dot H2O) single crystals have been grown from an aqueous solution of potassium hydrogen phthalate enriched with equimolar concentration sodium hydroxide at room temperature. Single crystal XRD analysis reveals that the mixed crystal belongs to the trigonal system with space group P3(-)1c and the cell parameters are, a=16.7228(2) angstrom, b=16.7228(2) angstrom, c=18.2742(5) angstrom, alpha=beta=90 degrees, gamma=120 degrees, nu=4425.75(14) angstrom(3) and Z=12. The partial substitution of cation results in defect centers which influence the physical properties. The co-existence of potassium and sodium ions in the crystal is further confirmed by inductively coupled plasma, atomic absorption spectroscopy and energy dispersive X-ray spectroscopy. The vibrational patterns in FT-IR are used for identifying the material and the thermal study by thermogravimetry/differential thermal analysis indicates the stability. (C) 2013 Elsevier B.V. All rights reserved.