[en] Sellafield nuclear fuel reprocessing plant has been discharging ¹⁴C as a component of the low level liquid radioactive waste into the Irish Sea since 1952. This ¹⁴C is of radiological importance due to its long half-life, mobility in the environment and propensity for entering the food chain. Unlike the overall tend towards lower discharge activities since the peak in discharge activity during the 1970s (for example ¹³⁷Cs and ²⁴¹Am), the discharge activities for ¹⁴C have increased since 1994/95 due to a change in discharge policy from mainly atmospheric discharges to mainly marine discharges. In contrast with other Sellafield derived radionuclides (for example ¹³⁷Cs and ²⁴¹Am), the behaviour of ¹⁴C in the lrish Sea is, however, poorly understood despite an increase in research effort in recent years. This study was therefore undertaken to improve our understanding of temporal variations in ¹⁴C specific activities in the biogeochemical fractions of the water column and biota in the Irish Sea and North Channel in relation to the Sellafield discharges. The project also determined a budget of Sellafield derived ¹⁴C in the sediments of the NE Irish Sea and measured the flux of this ¹⁴C from the Irish Sea. Before the contribution of Sellafield derived ¹⁴C to any carbon fraction can be made the activity of the marine background must first be estimated. The dissolved inorganic carbon (DIC) fraction of the water column, mussel, seaweed, crab, flatfish and roundfish samples were collected annually from the west coast of Co. Donegal, Ireland, and from these, the average current marine background activity was estimated to be approximately 253 Bq kg^-1 C. There were clear signs of incorporation of Sellafield derived ¹⁴C to the biogeochemical fractions (dissolved inorganic carbon or DIC, dissolved organic carbon or DOC, particulate inorganic carbon or PIC and particulate organic carbon or POC) of the water column in seawater from the NE Irish Sea (Nethertown and St. Bees Head site) and North Channel (Portpatrick). although ¹⁴C specific activities of the biogeochemical fractions of the water column in the North Channel were lower than those found close to the point of discharge. (author)