Freshwater and marine dissolved organic matter (DOM) is a complex mixture of chemical components that are central to many environmental processes, including carbon and nitrogen cycling. Due to its wide range of chemical-physical properties, the sampling and separation of its components is challenging, mainly due to coelution and irreversible sorption issues on traditional chromatographic columns. For this reason, questions remain as to its chemical characteristics, sources and transformation mechanisms. Here, we employ novel passive sampling techniques for isolation coupled with advanced analytical techniques for the characterisation of DOM from marine and freshwater environments. The spatial and temporal variation of DOM composition was investigated in a number of freshwater bodies along with the development and application of novel passive samplers within marine environments. In Chapter 2 we employ 1- and 2-D nuclear magnetic resonance (NMR) spectroscopy to investigate the structural components of lacustrine DOM from Ireland, and how it varies within a lake system, as well as to assess potential sources. Major components found, such as carboxyl-rich alicyclic molecules (CRAM) are consistent with those recently identified in marine and freshwater DOM. Lignin-type markers and protein/peptides were identified and vary spatially. Phenylalanine was detected in lake areas influenced by agriculture, whereas it is not detectable where zebra mussels are prominent. The presence of peptidoglycan, lipoproteins, large polymeric carbohydrates and proteinaceous material supports the substantial contribution of material derived from microorganisms. A major challenge in environmental chemistry at the moment is finding materials that can isolate all components of DOM from both fresh and marine water. In the freshwater studies of chapter 2 a cellulose sorbent was used to isolate the DOM from water. However, cellulose use is not possible in marine water studies as the Cl- ions compete for binding sites affecting the sorption of DOM. Therefore activated carbon was investigated as a possible sorbent of DOM within marine environments in chapter 3. Activated carbon passive samplers showed an steady uptake of organic carbon over time, however NMR results were inconclusive as major DOM signals CRAM and MDLT were absent from 1H spectra. Consequently, cation exchanged monmorillonite clays were investigated as sorbents and showed very good potential, at least in the sorption of the aliphatic component of DOM. After a 28 deployment within marine and freshwater environments GCMS analysis identified a large aliphatic component sorbed to the clay. In addition sterols and sugars were also identified in the DOM matrix. To further investigate clay as an isolation medium for DOM it was included in a more in-depth study of water chemistry in the Shannon Pot, Co Cavan, Ireland (Chapter 4). The DOM composition and hydrochemistry within this karst aquifer was investigated using: 1. clay passive samplers followed by GCMS TMAH chemolysis, 2. NMR and 3. water quality indicators over a fifteen month period. This data was correlated with rainfall records. Phosphate and dissolved oxygen levels exceeded recommended concentrations at times of high precipitation indicating that fertiliser use is influencing the water chemistry at this important site. These events were also evident in DOM chemistry as an increase in biopolymers such as lignin was observed during the same periods of increased precipitation. Furthermore, evidence for anthropogenic influence in the surrounding landscape was found in the DOM including herbicides that may be more stable in the environment than previously thought. The results indicate than the DOM composition and its hydrochemistry is strongly influenced by the hydrological events such as rainfall.