Dissolved organic matter (DOM) in seawater and freshwater represents a carbon reservoir comparable to atmospheric CO2 (respectively 624 and 750 gT). Despite being tightly related, as CO2 represents a primary product of DOM mineralization, DOM still remains largely uncharacterized, containing various classes of compounds in concentrations ranging from picomolar to micromolar. Due to the complexity of DOM, conventional liquid and gas chromatographic analysis are inhibited due to extensive co-elution. In order to overcome the analytical challenges presented by DOM, this project proposes alternative multi-dimensional chromatographic approaches to fractionate and isolate single compounds from this organic pool. Firstly, size exclusion chromatography (SEC) was coupled to reversed-phase liquid chromatography tandem mass spectrometry (RP-LC-MS/MS) to fractionate DOM constituents in terms of size and polarity. Seven fractions were collected from the first dimension, allowing the isolation of up to 142 single compounds from the second. To fractionate DOM according to the polarity of its components, high-performance counter current chromatography (HPCCC) was chosen as an alternative first chromatographic dimension, leading to the separation of five fractions which were also further processed by RP-LC-MS/MS. High resolution MS (HR-MS) generated exact mass losses that occurred within the isolated compounds, representing a further step towards the identification of DOM constituents.Lastly, ion-exchange chromatography with amperometric detection (IEC-PAD), was applied to demonstrate the presence of analogous classes of compounds within naturally occurring and artificially produced DOM samples (ADOM), showing that the bulk of DOM is probably of microbial origin and providing the separation of various classes of neutral sugars within DOM.