The effect of sedimentary microbial communities on the composition of pentacyclic triterpenol markers as well as b-sitosterol was studied in mangrove surface sediments of North Brazil during a three-month degradation experiment. Leaves of Rhizophora mangle, Avicennia germinans and Laguncularia racemosa were incubated with surface sediment and analysed by GC/MS for b-sitosterol, taraxerol, b-amyrin, germanicol, lupeol and betulin. Taraxerol showed the strongest resistance against microbial degradation and clearly increased up to 68% of the sum of the selected markers. Betulin was degraded completely after 40 days, most likely due to its higher polarity. By means of a numeric model the marker composition in surface sediments was explainable as a combination of the dominant input of R. mangle leaves and selective microbial degradation. Calculated marker degradation constants from the model were compared with measured constants in the experiment. On the basis of calculated constants the average age of the organic matter in the upper 1.5 m of sediment was approximately 530 yr.