The paper compares building and decomposition pathways of two phosphonic acids, amino trimethylene phosphonic acid (ATMP) and 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP). The theoretical formation reactions were composed using elementary reactions and compared to reaction routes published in literature. As result, summary reaction pathways for both phosphonic acids are proposed which only vary in the number of reaction steps required. detpmp These reaction steps include carbonyl reactions, SN2-reactions, and ionic reactions. The synthesis of ATMP proceeds in three reaction steps, whereas HEDP is formed in one reaction. The thermal decomposition of both phosphonic acids in solid state was examined by combination of thermogravimetry coupled with infrared spectroscopy as well as pyrolysis gas chromatography coupled with mass spectrometry. Decomposition mechanisms were deduced and compared to the theoretical findings resulting in the conclusion that the decomposition processes of ATMP and HEDP follows the formation mechanism. Thus, the suitability of theoretical considerations for the understanding of thermal decomposition processes is shown.