Marine dissolved organic matter (DOM) represents an enormous amount of organic carbon comparable to that of atmospheric CO2. A major part of DOM is refractory and resistant to chemical decomposition and despite manifold analytical approaches the bulk of its chemical structure remains completely unknown. This study addressed the question: What are the sources and formation processes of DOM in the ocean? The Southern Ocean is a key area to study DOM of pure marine origin. However a considerable amount of terrestrial DOM is transported by rivers to the oceans. The modification of terrestrial DOM by photo-degradation was examined in a 10-day degradation experiment. Software was developed to process chemical formulas for several thousand mass peaks in the FTMS spectra. On this basis the elemental contribution of dissolved oxygen and hydrogen to DOM was estimated. Selective losses of compounds with high O/C and low H/C ratios indicate loss of aromatic compounds and carboxylic acids during photo-degradation. The chemical formulas varied significantly between marine and terrestrial DOM sources and between marine DOM samples of differing maturity states.