Herein we report on the photocatalytic degradation of natural organic matter from a bog lake (Lake Hohloh, Black Forest, Germany) as followed by size exclusion chromatography with dissolved organic carbon (DOC) and ultraviolet (lambda = 254 nm) detection (SEC-DOC and SEC-UV). When irradiating this humic-rich water in the presence of 0.5 g L-1 titanium dioxide (P25),we found a preferential degradation of the higher molecular weight DOC fraction. This was coupled to growth of the middle and small DOC fractions. Mineralization proceeded in two distinct steps: a first stage without noticeable mineralization, followed by a phase of steady decrease in dissolved organic carbon. Formic, oxalic, succinic and glutaric acids were found in the irradiated samples and contribute significantly to the bioavailable DOC after irradiation, which favors bacterial regrowth. (C) 2008 Elsevier B.V. All rights reserved.