We used solid-state NMR spectroscopy to gain detailed information about the proton positions, proximities and the hydrogen bonding network in the environmentally-friendly flame retardant melamine orthophosphate. High-resolution proton one- and two-dimensional solid-state NMR spectra were obtained at high external magnetic field in combination with fast magic-angle-spinning of the sample. Furthermore, we recorded homo- and heteronuclear correlation spectra of types 15N–15N, 1H–13C, 1H–15N and 1H–31P. In addition we determined the geometry of the NH and NH2 groups in melamine orthophosphate by 15N–1H heteronuclear recoupling experiments. We were able to completely assign the different isotropic chemical shifts in MP. Furthermore we could identify the protonation of the melamine and orthophosphate moieties. The experimental results are discussed in connection with the structural model obtained by powder X-ray diffraction together with a combined molecular modeling-Rietveld refinement approach [De Ridder et al. Helv. Chim. Acta 87, 1894 (2004)]. We show that the geometry of the NH2 groups can only be successfully estimated by solid-state NMR.