Formation and stability of Co(II) chlorocomplexes were studied using UV-Vis and near Infra Red spectrophotometry in methanol at 25°C. Formation of three chlorocomplexes and a structural modification Oh →Td were obtained from the study of the equilibrium: Co 2+ + j Cl-CoClj (j-2)-. The stability of the monochlorocomplexe was studied by the graphical method of Kruh and the stability constants were calculated with the SIRKO program based on the least-square method, the retained model for the set of all solutions was [1, 2, 3]. Water effect on the stability of the formed complexes was carried out from recorded spectra analysis for different water fractions xwater.