Two electroanalytical techniques, direct potentiometry with a copper ion selective electrode (ISE) and potentiometric stripping analysis (PSA), were used to evaluate the lability of copper-f&k acid complexes (Cu-FA) in 0.5 mol/l sodium perchlorate and in synthetic sea water. The determinations were performed at pH 6.5, and for sea water also at pH 7.5. By ISE potentiometry, in 0.5 mol/l sodium perchlorate, the FA total complexation capacity (CC) in the l-150 mg/l FA concentration range, as well as the 1: 1 Cu-FA conditional stability constants for two different types of binding sites (K, and K,) and the respective metal-binding capacities (C, and C,; Ci + C, = CC) were obtained. It was found that CC was approximately constant over the FA concentration range studied: 1.81 mmol/g of FA or 5.2 mmol/g of total organic carbon (TOC). For 0.2-18 1 FA I t/ 10.1 1 t molar ratio range, the values of the complexation parameters were as follows: log K, = 5.77 f 0.09, C, = (4.7 f 1.2) x lop5 mol/l and log K, = 4.22 f 0.10, C, = (14.8 f 1.7) x lo-' mol/l (95% confidence limits, 20ยฐC). These parameters were also obtained at 25ยฐC. K, and K, decreased with the decreasing of IFA I JlCu I t. The labile copper fraction was determined by PSA, in a large 1FA It/ 1Cu 1 t molar ratio range (89-1.6) in 0.5 mol/l sodium perchlorate medium, at -0.5 V vs. SCE and 20ยฐC. The results were compared with the inorganic copper fraction obtained by a computational simulation based on equilibrium considerations. The lowest lability (dissociation of ca. 8% of the Cu-FA complexes) was found for the highest I FA 1 t/ 1 Cu 1 r ratio, which corresponded to 1 Cu-FA I / TOC = 0.06 mmol/g. The lability increased up to ca. 61% when lFA 1 1/ lCu 1 t decreased down to 6 (or ICu-FA I /TOC = 0.8 mmol/g) and slightly decreased down to ca. 50% for lower 1FA 1 ,/lCu 1 t values (the lowest was 2 mmol/g). In synthetic sea water, the inert copper fraction (i.e., 1 Cu I t/ I Cu I labile measured by PSA) was not very different from that found for 0.5 mol/l sodium perchlorate for identical 1FA It/ 1Cu 1 t at pH 6.5. Similar results were obtained at pH 7.5. These results suggest that the PSA kinetic behaviour of FA in sea water may be not very different from that observed for the simpler medium of similar ionic strength.