Distribution of suspended matter and its chemical composition were investigated in the Sangga Besar River Estuary in Matang Mangrove Forest, Malaysia. The C, N contents were higher in the upper part of the river and decreased towards the river mouth. Phosphorus content was as high as 129 μmol/g in the fresh water area, in which inorganic phosphorus accounted for about 70% of the total. Potentially bio-available phosphorus extracted by the citratedithionite-bicarbonate procedure (CDB-P) was the major component of inorganic phosphorus. Phosphorus content in the suspended matter decreased linearly with salinity, reflecting the process of CDB-P release from suspended matter into the estuarine water during the transportation to the sea. The average C: N: P atomic ratio of the organic substances contained in the suspended matter in the estuary was estimated to be 140 : 16 : 1, which is considerably different from the ratios for mangrove litter and rather similar to the Redfield ratio, indication the higher contribution of living microorganisms than that of the mangrove litter.