Oleic acid labeled with radioactive carbon at the carboxyl group (oleic acid-1-C-14) has been applied to follow the hydrotreating process (reactions) of this compound over Mo/P/Al2O3 and NiW/Al2O3 catalysts, treated with H2S. The aim of the present study was to expose the transformation of the carboxyl group, including its splitting from the oleic acid, on two catalysts of substantially different chemical content. A flow-system catalytic reactor equipped with a special unit for microanalysis was applied for the study of the decarboxylation, decarbonylation and hydrodeoxygenation reactions at atmospheric pressure in the stream of hydrogen gas of different rates in the temperature range of 573-673K. The gas products contained only (CO)-C-14, (CO2)-C-14 and (CH4)-C-14. One of the alkanes among the products formed in the hydrodeoxygenation process (C18H38) was also radioactive. No radioactive products were detected among the alkanes with carbon number <18. The ratio of Sigma C-1 formation the Sigma C-14(1) products from the oleic acid conversion was lower, than that of the (C17H35CH3)-C-14 produced. This is explained by the higher energy barrier, i.e. activation energy value, required for the dissociation of the C-C bond on the catalysts-being an inevitable condition of the formation of C-14(1)-products-than that of the C-14-O bond dissociation on these catalysts, required for the transformation of (COOH)-C-14 + 1/2H(2) -> (CO2)-C-14 + H2O. It is stated that at the applied conditions, only small amounts of sulfur leave the S-35-labeled sulfided catalyst samples, consequently the sulfur loss of the samples can influence the catalytic behavior of these samples only after a longer period. The experiments serve for a general conclusion: the radioactive microanalytical method based on the application of oleic acid-1-C-14, is suitable for testing and evaluation of new catalyst samples in hydrotreating process of natural triglycerides. (C) 2014 Elsevier B.V. All rights reserved.