Sterol, pigment and elemental carbon and nitrogen compositions are reported for surface suspended particulate materials collected dong 1200 Km reach of the Ubangui and Congo rivers from Bangui to Brazzaville during the high flood period of 1989. Rock-Eval pyrolysis and amino-acid datas of monthly sampling at Brazzaville are also related. Particulate organic matter associated to surface suspended particles transported in the mainstem is mainly originated from soils and characterized by a low content of algal carbon (< 2 % of POC) evaluated by measurements of chlorophyll g concentrations. The ratios of cuticular plant sterols ( stigmastérol/sitostérol) of surface particles indicate rather homogenous input of organic matter from soils in the Ubangui river, whereas this ratio point out complese and heterogenous input of detritical organic matter in the Congo river. Some tributaries: Lobaye, Motaba and Sangha, Likouala, Alima, have been characterized by extreme acidic waters and by particulate organic matter almost esclusively from allochtonous origin.
