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Abstract. Sterols were analyzed in suspended particles collected in January 1991 in the Solo River system and in the Serayu River, Java, Indonesia. Free sterols were extracted from particles larger than 0.7 #im and analyzed, after derivatization into their trimethylsilyl esters, by GC and GC/MS. Concentrations of total sterols ranged from 438 to 7922 ng/l, or from 2.4 to 183.8 ng/mg of total suspendedmatter, which varied from 3.3 to 400 and 471 mg/I, respectively in the Serayu River and at the downstream station in the Solo River. POC concentrations also varied in a wide range, from 0.91 to 4.72 and 6.13% of TSM, respectively at the above stations, and were associated with sterol/POC values ranging from 0.15 to 1.75 ag/mg. Eleven structures of C27, C 28 and C 29 stenols and associated stanols were identified. 28A 22 was only found at downstream stations in the Solo River and in the Serayu River. This unique distribution, different from that of other C 27 , C 28 and C 29 sterols, suggests a predominantly autochthonous origin for these compounds associated with an increased planktonic biosynthesis near the estuary. Concentrations of 28A 5 , 29A 5 ' 2 and 29A 5 showed similar spatial distributions and increased downstream, reflecting the significant accumulation of organic matter originating from the vegetation of the various drainage basins.Values of the autochthonous versus terrigenous sterol index, defined as 27A 5 /29A 5 ' 2 2 + 29A 5 were in the 1.4-1.9 range at upstream stations, whereas at downstream stations lower values were found, 0.4-0.6, which also corresponded to higher concentrations of TSM and lower POC values.Insofar as the stanol/stenol values can be used to estimate the bacterial activity of oxic waters, simultaneous variations of C27 and C 29 stenol/stanol pairs suggest rather different bacterial degradation capacities of autochthonous versus allochthonous organic matter. The wide differencies between the values of the stenol/stanol pairs observed in one of the main tributaries and in downstream stations of the Solo River is evidence that allochthonous organic matter is much more resistant than autochthonous matter. The low index value observed in the Serayu River indicates the highly refractory nature of both autochthonous and allochthonous organic material.