Water-soluble wheel-like icosanuclear peroxotitanate K-16[Ti-20(mu-O)(8)(HO2)(8)(O-2)(12)(R,R-tart)(12)].52H(2)O (1) chelated by tartrate has been successfully isolated. As the largest peroxotitanate reported, {Ti-20} features 20 (hydro)peroxo groups with three kinds of coordination modes in mu-eta(1):eta(2), mu-eta(2):eta(2) and eta(2) fashions. The cluster is stable in solution and solid states. It has been tested for the catalytic oxidations of methyl phenyl sulfide and pyridine with hydrogen peroxide, respectively, which shows reversible elimination and the addition of peroxo groups. This provides a rare example of well characterized titanium peroxide for homogeneous catalysis and mechanism research.