Solid and dissolved organic matter (DOM) compositions were investigated in a pristine and a deforested tropical peat forest in Brunei Darussalam. A combination of elemental (%C, %N, C/N), isotopic (δ15N, δ13C, Δ14C), molecular (lignin phenol biomarkers) and optical (Specific UV Absorbance at 280 nm (SUVA280), fluorescence index (FI)) analyses were performed to characterize DOM in porewater and river water. The DOM composition was compared to vegetation and peat to assess DOM origin and transformations in the ecosystem. Significant relationships were observed between optical properties (SUVA280, FI) and bulk (Δ14C) and molecular (ratio of cinnamyl to vanillyl phenols, C/V) composition of DOM.