The present study aims to determine the compound-specific δ13C in n-alkanes from marine sediment samples from Guanabara Bay (Rio de Janeiro/RJ) and leaf samples of 3 plant species typical from mangrove forests collected in different regions of Brazil. Methods were tested to remove the interfering compounds and isolate the class of n-alkanes without any losses in the analytical quality. The results were satisfactory for the methods of silica impregnated with AgNO3 and urea adduction, which allowed further analysis of the δ13C n-alkanes by GC-C-IRMS. The δ13C (C15-C40) between -34.2 and -25.5 % for sediment suggest multiple sources and are within the same range of other contaminated environments. The most depleted δ13C of n-alkanes and bulk for P6, P7 and P8, associated with CPI, suggest a prevalence of biogenic n-alkanes from terrestrial origin for these stations in relation to others. Small isotopic differences between the odd and even-numbered chains and enrichment in long-chain homologues indicate a higher petrogenic hydrocarbons contribution to the sediments of the bay. Regarding the leaves, CPI and ACL ratios, as well as principal component analysis for concentrations of n-alkanes indicate that the distinction between the samples is given most by intraspecific factors of species than by the location where plants developed. More negative δ13C (C25-C35) values for Rhizophora mangle, which usually occupies sites with more presence of water, and less negative values for Laguncularia racemosa, species commonly found in places where the flood tide is less frequent, may be linked to the specific characteristics of each one with respect to the water use efficiency.