Reversed-phase liquid chromatography/mass spectrometry (LC/MS) is introduced as a new molecular fingerprinting technique for tracing terrigenous dissolved organic matter (DOM) and its photochemical decay in the ocean. DOM along a transect from the mangrove-fringed coast in Northern Brazil to the shelf edge was compared with mangrove-derived porewater DOM exposed to natural sunlight for 2-10 days in a photodegradation experiment. DOM was isolated from all samples via solid-phase extraction (C18) for LC/MS analysis. DOM in the estuary and ocean showed a bimodal mass distribution with two distinct maxima in the lower m/z range from 400 to 1000 Da (intensity-weighted average of 895 Da). Terrigenous porewater DOM from the mangroves was characterized by a broad molecular mass distribution over the detected range from 150 to 2000 Da (intensity-weighted average of 1130 Da). Polar compounds, i.e., those that eluted early in the reversed-phase chromatography, absorbed more UV light and had on average smaller molecular masses than the more apolar compounds. After 10 days of irradiation (∼ 70 kWh/m2), mangrove DOM resembled open-ocean DOM in its mass distribution (intensity-weighted average of 885 Da). In addition, a large fraction of UV-absorbing compounds, which were present in mangrove samples but absent in offshore samples, were not detected after photodegradation. However, the bimodal mass distribution in ocean waters was not reproduced during photodegradation. This mass distribution is certainly a reflection of specific molecular properties of marine DOM which systematically differed from the mangrove samples. The molecular patterns of DOM in the ocean did not show significant contribution of terrigenous DOM, which is in contrast to previous stable carbon isotope analysis. If, however, photochemical modifications of the terrigenous component are considered as an additional mechanism besides simple mixing of two end members (marine and mangrove), the observed molecular patterns of oceanic DOM are consistent with the contribution of terrigenous DOM in these samples. In order to explore the molecular mass information, the mass spectra of the different samples were compared through multivariate statistics. Cluster analyses and multi-dimensional scaling (MDS) revealed significant differences between mangrove and oceanic DOM that successively disappeared in the course of the photodegradation experiment. With help of discriminant analyses, the similarity between photodegraded mangrove DOM and open-ocean DOM could be confirmed on an individual m/z level. Though conventional ion trap mass spectrometry (resolving power of ∼ 1000) does not resolve the complexity of DOM at the level of single molecules, it does provide detailed molecular fingerprints. This molecular fingerprinting technique provides a means to trace DOM and modifications in its molecular structure in aquatic systems. © 2007 Elsevier B.V. All rights reserved.