Definifing the stability of drugs is mostly reduced to determining the adverse properties caused by degradation products and contaminants that reduce the quality, safety and efficiency of the drug. In order to accurately determine the production process, as well as storage and transport conditions, it is important to examine how the quality of the pharmaceutical product changes under the influence of various environmental factors. Before commencing commercial use of pharmaceutically active substances, it is necessary, in addition to stability testing, to evaluate their properties which may have a significant effect on living organisms and the environment. Only after the tests have been performed, can the pharmaceutically active compounds enter the testing phase of potential implementation in new pharmaceutical products. One way to test stability is to subject the active substances to extreme chemical and environmental conditions, the so-called forced decomposition test. The aim of this study was to determine the structure and properties of degradation products of two 2-phenylbenzothiazole derivates. Samples of the compounds LD-4, IUPAC called 2-(4-amidinophenyl)-6-aminobenzothiazole chloride, and LD-5, IUPAC called 2-(4-aminophenyl)-6-(imidazolin-2-yl)benzothiazole mesylate, were tested under conditions of acid hydrolysis (0,5 M HCl), basic hydrolysis (0,5 M NaOH), peroxide oxidation (10 % H2O2), elevated temperatures (80 ˚C i 100 ˚C) and UV/VIS light (500 Wh/m^2), and product identification was performed using a high performace liquid chromatograph connected to a high resolution mass spectrometer (HPLC-QTOF). Based on the obtained mass spectra, the structure of degradation products was determined and the degradation pathway of basic compounds was proposed. Based on the specific structure of degradation products, the values of developmental toxicity, mutagenicity and bioaccumulation factor were estimated using T.E.S.T. software program.