A novel class of fused azaisocytosine-containing congeners, i.e. 3-(4-nitrophenyl)-8-(R-phenyl)-7,8-dihydroimidazo[2,1-c][1,2,4]triazin-4(6H)-ones, acting as anticancer antimetabolites, were subjected to the heating in order to evaluate their decomposition mechanism. The pyrolysis and oxidative decomposition course was studied with a use of simultaneous thermal analysis: thermogravimetry/differential scanning calorimetry (TG/DTG/DSC) coupled online with Fourier transform infrared spectroscopy (FTIR) and Quadrupole mass spectrometry (QMS). The performed analyses allowed to conclude that, the pyrolysis and oxidation processes of the tested compounds run through simultaneous, radical processes in one, main stage (inert conditions) or in two stages (oxidative conditions) resulting in the emission of NH3, RNCO, HCN, CO, CO2, NO2, aromatics and alkenes (inert atmosphere) and NH3, RNCO, HCN, CO, CO2, NO2, NO, N2O, H2O, aromatics and alkenes (oxidative atmosphere). The results confirmed the simultaneous breaking of Csingle bondC, Csingle bondN, Nsingle bondH, Csingle bondH and Csingle bondCl bonds in the starting compounds and the created intermediates, and then additional oxidation processes of some, formed products to the corresponding nitrogen oxides and water. The connection between the thermal behaviour of the tested compounds and their possible use as candidates for pharmaceutical applications was emphasized.