The thermal degradation of biologically active triazines, whose Cl substituent is bonded to the heterocyclic ring, is a multi-stage process. The final products are high molecular compounds. Investigations of these compounds were performed using the thermogravimetric method (TG) at the temperature range from ambient temperature to about 850 K using the Q technique and labyrinth crucibles. MALDI TOF spectrometry and elementary analysis were also used to confirm the degradation products' structures. The compounds identified after the measurement were identified as dealkylation products, compounds with removed amino group and high molecular weight products. (C) 2004 Elsevier B.V. All rights reserved.