The two main aims of this study were; (i) to determine the equilibrium fractionation effects of the stable isotopes of carbon between dissolved molecular CO2 (C02(aq) ) and bicarbonate (HC03-) (Eb/a ) and HC03 - and carbonate (CO/-) (Eclb ) in sea water, and (ii) to investigate the relationship between the isotopic composition of phytoplankton (013CpOC) and their photosynthetic source of inorganic carbon. To determine Eb/a and Eclb in sea water (S = -34) experiments were conducted at different pHs and two temperatures (SOC and 20°C). The measured values of Eb/a (SOC = 11.67 ± 0.34 %0 and 20°C = 9.48 ± 0.26 %0) agreed well with previous estimates derived in distilled water indicating Ebla is not modified by the ionic effects of sea water. The values of Ec/b determined during the same experiments were significantly larger than previous estimates (SOC = -8.1 ± 2.2 %0 and 20°C = -14.2 ± 2.4 %0). This may indicate that the measurement made during this study are in error. It is suggested that the error is either due to the relative imprecision associated with the determination of the isotopic composition of the total dissolved inorganic carbon pool or due to incomplete isotopic equilibrium within the experimental system. The relationship between changes in the o13Croc and the dissolved inorganic carbon pool was investigated during temporal studies of phytoplankton blooms in a mesocosm and the Menai Strait, North Wales. During the mesocosm experiment the isotopic enrichment between the phytoplankton and CO2(aq) (EPOC/a ) decreased from -- -10 %0 to - +0 %0 during the course of the bloom. The observed changes in 013CpOC are generally consistent with previously published empirical models which assume CO2(aq) is the exclusive source of photosynthetic inorganic carbon. This agreement, and the application of physiological models appeared to confirm that CO2(aq) is the most likely source of photosynthetic inorganic carbon. The o13Croc results obtained during two consecutive phytoplankton blooms in the Menai Strait, North Wales, were corrected for the presence of refractory organic matter to obtain the isotopic signal associated with the phytoplankton, 013Cp. It was apparent that the relationship between 013Cp and [C02(aq)] differed between the two blooms. During the first bloom ( diatomaceous), the observed relationship between 013Cp and [C02(aq)] was consistent with the uptake of CO2(aq). However, the 013Cp signal during the second bloom, dominated by Phaeocystis, was independent of the [C02(aq)J and therefore it was concluded that HC03- is the most likely source of inorganic carbon.