New examples of terminal group 5 chalcogenido-substituted polytungstates with the Keggin [PW(11)O(39)ME](4-) (M = Nb, Ta; E = S, Se) (pictured) and Lindquist [W5O18NbSe](3-) structures have been prepared by oxo/chalcogenido ligand substitution reactions. The compounds and their reactions with H2O in nonaqueous solution were characterized by IR and O-17, P-31, W-183,and Se-77 NMR spectroscopy. The terminal group 5 metal-chalcogenido bond, found previously to be very susceptible to hydrolysis, is hydrolysis resistant for the Keggin ion series (M = Nb; E = S, Se : M = Ta; E = S).