This work describes the synthesis and the thermoanalytical characterization of guanidinium-5-aminotetrazolate (GA). GA is a new nitrogen-rich energetic material. It is not mentioned in the chemical literature so far. The molecular structure of the compound has been determined by IR, H-1-, C-13- and N-15-NMR spectroscopy. The thermal properties, the decomposition pathways and its volatile products were investigated by thermal analysis and are discussed.