<p>The influence of the structure of potential medicines, <i>i.e.</i> methyl [4-oxo-8-(R-phenyl)-2,3,4,6,7,8-hexahydroimidazo[2,1-<i>c</i>][1,2,4]triazin-3-yl]acetates, on their melting points, thermal properties and decomposition paths has been assessed in inert and oxidizing conditions by the use of DSC and simultaneous TG/DTG/FTIR/QMS analyses. The melting points of particular molecules were directly dependent on their structure, and decreased as follows: the <i>meta</i>-Cl-substituted compound &gt; the <i>para</i>-Cl-substituted compound &gt; the parent compound (an unsubstituted structure) &gt; the <i>para</i>-OCH₃-substituted compound. The high thermal stability (above 240&#xa0;°C) of the tested compounds in both furnace atmospheres was proved. The thermal resistance of these compounds increased in inert conditions as follows: the parent compound &lt; <i>meta-</i> and <i>para</i>-Cl-substituted compounds &lt; the <i>para</i>-OCH₃-substituted compound, and decreased in oxidizing conditions as follows: the <i>meta</i>-Cl-substituted compound &gt; the <i>para</i>-Cl-substituted compound &gt; the <i>para</i>-OCH₃-substituted compound &gt; the unsubstituted compound. The pyrolysis process of the tested molecules resulted in the emission of CH₃OH, NH₃, HCN, HNCO, HCONH₂, CH₃CHO, aliphatic amines, alkenes, CH₃Cl (for Cl-substituted compounds), CH₄ (for the unsubstituted compound), aromatic compounds, CO₂, CO and H₂O. The oxidative decomposition process was connected with the formation of the same volatiles as during pyrolysis and additionally CH₃COOH, CH₃NO, NO and NO₂. These studies confirmed that the pyrolysis and oxidative decomposition processes of hydrogenated heterofused as-triazines with an ester functional group at the C3 proceeded by the radical mechanism in both furnace atmospheres employed.</p>