The presence of natural dissolved organic carbon (DOC) in drinking water supplies is undesirable due to its reaction with chlorine during water treatment and the resulting formation of disinfection by-products (DBPs) (Rook, 1976). Some of these compounds, principally trihalomethanes (THMs), have demonstrated carcinogenic properties and can potentially cause damage to the reproductive system (Bull, et al, 1995; Nieuwenhuijsen, et al, 2000). Concentrations of DOC in freshwater rivers and lakes in the UK have increased by 91% during the last 15 years (Evans, et aL 2005), with the rise attributed to the destabilisation of peatland soils through climate change, and the release of some of the vast carbon store of 455 Pg they have accumulated over the last few thousand years (Gorham, 1991). If the increase continues, it will create a serious challenge to water companies due to the need to minimise the formation of harmful DBP compounds. This project investigated the role of peatlands in the formation of THMs at selected reservoirs in north Wales. Water draining from Cors Erddreiniog fen into Cefni reservoir was found to be the most important source of DOC, comprising 33-57% of all the inputs to the lake on an annual basis; however, it was also demonstrated that sunlight can strongly degrade the DOC entering the lake from this source, thereby reducing its THMforming potential. Rapid growth of algae during the summer months contributes up to 45% of all the DOC inputs, although the lower molecular weight of the carbon compounds ensures that algae do not generate THMs as readily as terrestrially-derived DOC during water treatment. At Marchlyn Bach and Llugwy reservoirs, the DOC concentration of the lake water was lower, due to both the bog soils within the catchment releasing less DOC than the fen, and the much lower autochthonous production. The impact of rising temperatures on peat soil was to increase the leachability of DOC during spring for bogs and summer for fens, the latter being important for the Cefni as this is when THM concentrations are highest. Exposure to elevated concentrations of atmospheric ozone reduced the porewater DOC concentration of fen peat by as much as 55%, suggesting that through its effects on the carbon allocation within the above ground vegetation, elevated ozone could partially offset the predicted increasei n DOC leaching from fen ecosystemsth rough enhancedw arming and C02 concentrations.