The course of pyrolysis and combustion processes of four diethyl (2E)-2-{(2E)-[1-(R-phenyl)imidazolidin-2-ylidene]hydrazinylidene}butanedioates which act as anticancer agents was studied using simultaneous thermal analysis method (TG/DSC) coupled online with Fourier transform infrared spectroscopy (FTIR) and Quadrupole mass spectrometry (QMS) analyzers in helium and synthestic air atmospheres. It was found that the decomposition processes of the tested compounds in inert and oxidative conditions were complex and included some of the simultaneous reactions. The following decomposition reactions were observed: (i) pyrolysis of Csingle bondO, Csingle bondC and Csingle bondN bonds, partial decarboxylation and dehydration (ii) further partial fragmentation and pyrolysis of Csingle bondO, Csingle bondC and Csingle bondN bonds connected with decarboxylation and dehydration of intermediate products; (iii) simultaneous cracking of Csingle bondN and partial pyrolysis of formed aromatic-rich residue and additional oxidation processes in air conditions; (iv) fragmentation and pyrolysis processes (in inert conditions) and additional combustion processes (in oxidative conditions) of fused aromatic rings in connection with some chemical reactions between the decomposition fragments.