Poly(l-cysteine) (PLC) and 8-hydroxyquinoline (8HQ) were immobilized on controlled-pore glass and used in a flow injection system for the separation of Cd, Pb and Cu from synthetic sea-water, Co and Ni matrices as well as CRM sea-water. Both resins allowed for the quantitative recovery of 50 µg L ^–1 Cd and Pb in synthetic sea-water. However, low recoveries of 2-4% and 40-50% were observed using 8HQ for the separation of 50 µg L ^–1 Cd and Pb, respectively, from a 10 000-fold excess of Co and Ni, while PLC maintained quantitative recoveries. Neither 8HQ nor PLC showed reproducible or complete recoveries of Cu ²⁺ from the columns using the typical means for stripping (1 M HNO ₃ ). On-line breakthrough experiments showed that 8HQ had a significant strong binding site capacity for Cd, Pb, Cu, Co and Ni. PLC also had strong sites for Cd, Pb and Cu but showed only weak binding of Co and Ni. The selectivity of PLC against these harder acid metals allowed for quantitative recovery of Cd, Pb and Cu in Co and Ni matrices. Extracting low level spikes of Cd and Pb from CRM sea-water (CASS-1 and NASS-2) tested the application to ‘real’ samples. Recovery efficiencies of Cd were high for both CRM matrices studied. Pb recovery was good for CASS-1; however, recovery from NASS-2 was unexpectedly low. Mass transfer limitations were observed for both resins in the flow system, resulting in apparent decreased capacities at faster flow rates. Stability constants governing Cd adsorption by PLC and 8HQ were obtained by a non-linear least-squares regression analysis of the Cd binding data and revealed that at least four classes of binding site were present on both resins. Stability constants for the most stable sites were estimated using EDTA or ethylenediamine (en) as competing ligands. 8HQ had no sites that were competitive with EDTA, whereas PLC had an EDTA-competitive site with a stability constant of 1×10 ¹³ and a capacity of 1 µmol g ^–1 . Both PLC and 8HQ had sites that were stronger than Cd(en) ₂ with estimated stability constants ranging from 10 ⁹ to 10 ¹¹ . Weaker sites on the resins had stability constants that ranged from 10 ⁴ to 10 ⁶ . Cd was used to demonstrate the viability of this method for stability constant determination as it is well characterized for both 8HQ and PLC.