Single crystals of melaminium phthalate (MPH) were grown from aqueous solution by slow solvent evaporation method at room temperature. X-ray powder diffraction analysis confirmed that the MPH crystal crystallises in orthorhombic system with space group Pbcn and the calculated lattice parameters were a = 33.559 ± 0.066 Å, b = 10.517 ± 0.057 Å, c = 7.117 ± 0.025 Å, α = 90o, β = 90o, γ = 90o and V = 2511.94 (Å). The vibrational frequencies of various functional groups present in the MPH crystals have been derived from FIIR and FT-Raman analyses. The chemical structure of MPH was established by proton NMR and carbon NMR spectrum. The thermal decomposition behaviour of MPH has been studied by means of themogravimetric analysis at three different heating rates 10, 15 and 20oC min–1. The values of effective activation energy (Ea) and pre exponential factor (ln A) of each stage were calculated by model free methods: Arrhenius, Flynn-Wall, Kissinger and Kim-Park method. A significant variation of effective activation energy (Ea) with conversion (α) indicates that the process is kinetically complex. The linear relationship between the lnA and Ea values was well established (compensation effect). No detectable signal was observed during second harmonic generation test.