The equilibrium distribution of species formed between Cu(II) and N-acetylneuraminic (sialic) acid (I, LH) at 298 K has been determined using a two-dimensional (2D) simulation analysis of electron paramagnetic resonance (EPR) spectra. In acidic solutions (pH values < 4), the major species present are Cu2+, [CuL](+) [log beta = 1.64(4)], and [CuL2] [log beta = 2.77(5)]. At intermediate pH values (4.0 < pH < 7.5), [CuL2H-1](-) [log beta = -2.72(7)] and two isomers of [CuLH-1] [log beta (overall) = -3.37(2)] are present. At alkaline pH values (7.5 < pH < 11), the major species present is [CuL2H-2](2-), modeled as three isomers with unique g(iso) and A(iso) values [log beta (overall) = -8.68(3)]. Two further species ([CuLH-3](2-) and [CuL2H-3](3-)) appear at pH values > 11. It is proposed that [CuL]+ most likely features I coordinated via the deprotonated carboxylic acid group (O-1) and the endocyclic oxygen atom (OR) forming a five-membered chelate ring. Select Cu(II)-l species of the form [CuLH-1] may feature I acting as a dianionic tridentate chelate, via oxygen atoms derived from O-1, O-R, and one deprotonated hydroxy group (O-7 or 08) from the glycerol tail, Alternatively, I may coordinate Cu(II) in a bidentate fashion as the tert-2-hydroxycarboxylato (O-1,O-2) dianion. Spectra predicted for Cu(II)-l complexes in which I is coordinated in either a O-1,O-R {l(1-)} or O-1,O-2 {I(2-)} bidentate fashion {e.g., [CuL](+) (O-1,O-R), [CuL2] (bis-O-1,O-R), [CuLH-1] (isomer: O-1,O-2), [CuL2H-1](-) (O-1,O-R; O-1,O-2), and [CuL2H-2](2-) (isomer: bis-O-1,O-2)} have "irregular" EPR spectra that are ascribed to the existence of Cu(II)-I (monomer) reversible arrow Cu(II)-I(polymer) equilibria. The formation of polymeric Cu(II)-I species will be favored in these complexes because the glycerol-derived hydroxyl groups at the complex periphery (O,O-7,O-8(9)) are available for further Cu(II) binding. The presence of polymeric Cu(II)-I species is supported by EPR spectra( data from solutions of Cu(II) and the homopolymer of I, colominic acid (I-poly). Conversely, spectra predicted for Cu(II)-l complexes where I is coordinated in a {I(2-)} tridentate {e.g., [CuLH-1] (isomer: O-1,O-R,O-7, or O-8) and [CuL2H-2](2-) (isomer: bis-O-1,O-R,O-7, or O-8)} or tetradentate fashion {I(3-)} {e.g., [CuLH-3](2-) (O-1,O-R,O-8,O-9)} are typical for mononuclear tetragonally elongated Cu(II) octahedra. In this latter series of complexes, the tendency toward the formation of polymeric Cu(II)-l analogues is small because the polydentate I effectively wraps up the mononuclear Cu(II) center. This work shows that Cu(II) could potentially mediate the chemistry of sialoglycoconjugate-containing proteins in human biology, such as the sialylated amyloid precursor protein of relevance to Alzheimer's disease.