We examined the compositions of dissolved, fine and coarse particulate organic matter fractions (DOM, FPOM and CPOM, respectively) from 18 river sites in Peru along a 2000 km transect ranging from diverse Andean headwater environments, to depositional reaches, to the confluence of major lowland rivers that form the Rio Amazonas proper. The objective of the study was to evaluate the extent to which compositions of the three primary OM fractions evolve downstream, with the overall goal of assessing the relative effects of various processes in the dynamics of OM within a large river system. Composition was assessed by concentration, elemental (%OC, %N, C/N), isotopic (13C, 15N), hydrolysable amino acid, lignin phenol and mineral surface area analyses. Similar to previous results from the lower Amazon and from Bolivian tributaries, CPOM, FPOM and DOM showed distinct compositional differences from one another. However, compositions of OM size fractions at Andean sites were substantially different from lowland sites, with a clear downstream evolution in most OM properties toward typical lowland Amazon values. Andean FPOM and CPOM both had very high %OC and amino acid content, and low C/N typical of lowland FPOM. Andean UDOM showed low %OC, low C/N, high %TAAC and low non-protein amino acid content - also typical of lowland FPOM. These properties have been shown to be affected by selective partitioning onto minerals [Aufdenkampe, A.K., Hedges, J.I., Richey, J.E., Krusche, A.V., Llerena, C.A., 2001. Sorptive fractionation of dissolved organic nitrogen and amino acids onto fine sediments within the Amazon Basin. Limnology and Oceanography 46 (8), 1921-1935]. In contrast, lignin phenol acid to aldehyde ratios ((Ad/Al)v), indicators of diagenesis, were invariant and within typical lowland values over the entire transect. Thus, we propose that differences in the extent of organo-mineral association are the most plausible explanation for these trends. In the Andes, sand-sized particles appear to be stable aggregates of fine organo-mineral complexes and Andean DOM appears to be complexed with ultra-fine inorganic colloids. Therefore, unlike in previous studies, size was not always a good proxy for the degree of mineral association. However, it appears that selective partitioning of organic carbon and nitrogen molecules may be a dominant process in controlling OM composition in these rivers. © 2006 Elsevier Ltd. All rights reserved.