The aim of the present work is the study of the complex organic matter present in sediments from two retention ponds, Wissous located in an urban area and Chevire ´receiving road runoff. The main questions are relative to the origins, the transformation processes and the final structure of organic matter. The sediment from Wissous contained 22.3 wt.% of organic matter, compared to 14.7 wt.% for the Chevire śediment. FT-IR spectra did not reveal significant differences. Total organic matter (TOM) was fractionated into lipids and humic-like substances. ''Humic acid'' and ''humin'' fractions from the two samples were studied using pyrolysis techniques in presence of alkylating agents. The nature of TMAH pyrolysis products from Wissous and Chevire ´''humic acids'' did not differ. The presence of C 7 -C 28 aliphatic monocarboxylic acid methyl esters with a dominant short <C 20 mode (C 16 major) containing iso and anteiso C 15 , C 17 acids, and a minor C 20 -C 28 mode indicates a mixed bacterial and vegetal contribution. Pyrolysis of ''humin'' afforded the same kinds of products. The bacterial contribution with short acids and typical branched iso and anteiso C 15 and C 17 acids was dominant for all the samples. Nevertheless the plant contribution appeared to be somewhat more pronounced in ''humic acids'' than in ''humin'' for Chevire ´, and in humin for Wissous. Pyrolysis in presence of TEAAc allowed to distinguish three origins for the fatty acid methyl esters produced after TMAH pyrolysis of ''humic acids'' and ''humins''. The presence in the TEAAc products of fatty acid methyl esters (identified as methyl esters) showed that plant fatty acid methyl esters were trapped in the matrix, particularly for Wissous. Short acids including bacterial iso and anteiso C 15 , C 17 acids and identified as ethyl esters were present as trapped acids R-COOH. The last part which cannot react with TEAAc corresponds to acids which are bound to the matrix by ester groups and thus incorporated to the macromolecular structure.