Properties of the polymer metal complexes can be explained up to certain extent by the molecular structure of the complexes, but this área has not been well studied in the case of the chitosan metal complexes. In this work, we are proposing that chitosan is bonded through the 4 nitrogen to copper by means of square planar geometry. The polymer requires some structural modifications in order to be arranged in that geometry. Two sites in the chitin were detected, one similar to chitosan and the other in a square planar tetrahedrically distorted arrangement, with three atoms of nitrogen and one of oxygen or two atoms of nitrogen and two of oxygen. ESR studies of the copper complexes by means of chlorine and nitrate as ion counter were carried out. Nitrate exhibits one type of retention site and, on the other hand, chlorine presents two types of retention sites. © 2009 Sociedad Chilena de Química.