The molybdenum(VI) complex formation equilibria of microcrystalline chitosan (MCCh) with three different degrees of deacetylation (DD = 0.750, 0.812, and 0.948) were studied by pH-potentiometric and UV-spectrophotometric methods. The hydrolysis constants of Mo(VI) were evaluated under exactly the same conditions as during the experiments in presence of chitosan. It has been shown that the complexation model depends on DD. Besides the formation of a ligand to metal 2 : 1 MoO2B22+ complex ("bridge model") with A = MoO42-, B = MCCh, H = proton, the 1 : 1 MoO3B species ("pendant model") has been confirmed as well for the two lower values of DD and moderate excess of ligand. At the highest DD, the results indicate only the occurrence of 2 : 1 species. When taking into account the number of protons in reaction as the third index, the corresponding average overall stability constants log beta(abh) of MoO3B and MoO2B22+ reach log beta(112) = 13.8 and log beta(124) = 27.9, respectively. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114:1619-1625, 2009