Features of various Fe-containing nanosized slurry Fischer-Tropsch catalysts, obtained in situ by thermolysis, were investigated experimentally (catalytic tests, XRD, hermomagnetometric methods) and theoretically (DFT ab initio calculations, regression analysis). XRD patterns of the samples reveal the presence of preferentially single Fe7C3, Fe5C2 and Fe2C. A quantitative correlation was found between the values of the most important hydrocarbon group selectivity at 260 degrees C and the residual magnetization of the spent samples, regardless of promoter nature and amount. The Fe7C3 model clusters, according to the DFT results, prefer to be in high-spin states and substantiate the preference of associative coordinated CO. Because the CO associative sorption is unfavourable for chain growth on Fe Fischer-Tropsch catalysts and the carbides have Curie points <260 degrees C, the presence of nanosized paramagnetic carbide particles has been suggested to cause the abovementioned correlation. (C) 2019 Elsevier Inc. All rights reserved.