Abstract: Flow injection analysis with chemiluminescence detection (FIA-CL) via the reaction of copper with 1,10-phenanthroline was used for the determination of copper complexation in seawater. A detection limit of 0.1 nM Cu in undiluted seawater without sample pre-concentration was obtained. Copper speciation was assessed by titrating natural seawater ligands with added copper. This method was used to measure copper complexation throughout the water column in Monterey Bay, California. Two ligand classes were identified: a strong ligand (L1), ranging in concentration. from 1.3 to 3.1 nM, and a weaker ligand (L2), ranging from 9.4 to 25.6 nM. The average conditional stability constant for the strong ligand was log K1' 13.1±0.4 and for the weaker ligand log K2' 9.4±0.6. These measurements are in good agreement with complexation studies previously performed using established methods. This technique may eventually be adapted for in situ complexation measurements that can be used to monitor natural spatial and temporal variations in copper complexation, as well as coastal ecosystem responses to urban and industrial change.