Changes in the chemical nature of the water-soluble fraction (WSF) of crude oils caused by photolysis and heterogeneous photocatalysis using TiO2 were investigated by gas chromatography-mass spectrometry (GC-MS) and ultraviolet fluorescence spectrophotometry (UVF). Two Brazilian crude oil samples with different concentrations of WSF (45 and 15 mg Cl-1) were studied. Photochemical driven reactions were investigated under UV-Vis irradiation in the absence and in the presence of the photocatalyst. The presence of aromatic hydrocarbon in the WSF before irradiation, resulting from crude oil slick solubilization on seawater, was confirmed by chromatograms that showed a series of low molecular weight aromatic hydrocarbons together with a unresolved complex mixture (UCM). However, in the irradiated WSF these compounds were not detected. After 6 days photolysis period, WSF from both crude oils did not show peaks of aromatic compounds, whereas the presence of long chains unsaturated hydrocarbons as well as sulfur compounds were detected. In contrast, when TiO2 was employed, complete photodegradation of both crude oil samples in the WSF occurred after very short light exposure periods (1-2 days). Quantitative data were also obtained and discussed here. The results reinforce photodegradation as an effective weathering process for the transformation of dissolved crude oil fraction, particularly in high solar radiation environments. Complete degradation in a short period of time was only made possible by heterogeneous photocatalysis. (C) 2002 Elsevier Science B.V. All rights reserved.15541699243252