Cupric carbonate and cupric bicarbonate complexation constants were determined in natural seawater and in a variety of synthetic media. The formation constants of CuHCO3+, CuCO30 and Cu(CO3)22− at 25°C and zero ionic strength are: log βH0 = 1.8, log β10 = 6.82 and log β02 = 10.6. Formation constants of these species appropriate to 0.7 molar ionic strength and 25°C are log βH ∼- 1, log β1 = 5.73, log β2 = 9.3. Our results indicate that the inorganic speciation scheme of Cu(II) in seawater is dominated by CuCO30 and that the ternary species, CuCO3OH−, is of substantial importance.