This paper reports on the chemical structure and on some physicochemical properties of a new exopolysaccharide, CC-EPS, secreted by a filamentous nitrogen-fixing cyanobacterium, Cyanospira capsulata. Solution properties of CC-EPS were studied with the aim of characterizing the conformational state of the polysaccharide in water and in aqueous salt solution, in view of the peculiar rheological properties. Because the molecule has two different uronic acid residues, a variation of pH and/or ionic strength was considered to be the best perturbation to disclose any possible ordered structures. We found no evidence for a cooperative conformational transition from the change of the chiro-optical properties or from the smooth increase of pKa as a function of the degree of ionization, alpha, of the polyacid. The same conclusion was drawn from the study of the temperature dependence and ionic strength dependence (in the range up to 0.1 M NaCl) of solution properties. The data suggest that the solution conformation of CC-EPS is a random coil with a chain flexibility comparable to that of alginate.