A series of transition metal substituted polyoxotungstates (TMSP) were tested as catalysts for the oxygen delignification of eucalypt kraft pulp. These include Mn substituted polyoxotungstates of Keggin-type, alpha-[SiW((12-n))Mn-n(III) (H2O)(n)O(40-n)]((4+n)-) (n=1 (1) or n=2 (2)), sandwich-type alpha-B-[(PW9O34)(2)(Mn(4-n)MnnIII)-Mn-II(H2O)(2)]((10-n)-) (n = 1 (3) or n = 3 (4)), and the transition metal mono-substituted Keggin-type anions, such as alpha-[XWIIM(H2O)O-39](n-) (M = Co-III, X = Si (5) and X = B (6); M = Ru-IV, X = Si (7), and X=P (8)). All TMSP showed a catalytic activity. The experimental results revealed high selectivity of the residual lignin oxidation during oxygen delignification catalysed by 1-4, though the final kappa number of delignified pulps varied significantly. This was related to the removal of hexeneuronic acid residues at low pH diminishing the corresponding pulp kappa number counterpart. A comparison of the kappa number decrease and the delignification selectivity in conventional oxygen-alkaline bleaching process and in the TMSP-catalysed oxygen delignification showed significant advantages of the latter process. The best results, concerning both the delignification degree and the selectivity of delignification, were obtained with TMSP alpha-[SiW11MnIII.(H2O)O-39](5-) (1), alpha-B-[(PW9O34)(2)(MnMn3III)-Mn-II(H2O)(2)](7-)(4) and alpha-[SiW11CoIII(H2O)O-39](5-) (5). The application of catalysts 6-8 was limited by their structural instability under the delignification conditions. The practical aspects of the oxygen delignification catalysed by TMSP are discussed. 2005 Elsevier B.V. All rights reserved.