An empirical equation was proposed for calculating the chemical shifts of C-13 NMR signals from monomethylated alkanes and mono-n-alkyl-substituted cyclopentanes and cyclohexanes in the spectra of isoparaffin and naphthene fractions that form adducts with urea, depending on the analyte concentration in deuterochloroform C and the spectrum recording temperature T-rec. Deviations of the calculated chemical shifts from the measured values did not exceed 5 x 10(-3) ppm while varying T-rec from 303 to 323 K and C from 200 to 600 mg/ml. It was shown that such an accuracy is sufficient for the unequivocal identification of components by means of C-13 NMR technique; ignoring this dependence leads to errors in signal assignment in the spectra of petroleum fractions. The use of the chemical shift of certain signals as internal standards for determination of T-rec and C was proposed. It was assumed that numerical values of the temperature and concentration coefficients that describe the dependence of the chemical shift on T-rec and C would be useful for the C-13 NMR detection of new groups of compounds in petroleum hydrocarbons.