Zirconium containing mesoporous MCM-41 molecular sieves with Si/Zr ratios of 96 (A), 55 (B), 39 (C), and 23 (D) were synthesized and characterized by X-ray diffraction (XRD), surface area and pore size distribution measurements, and FTIR, diffuse reflectance UV-visible (DRUV-vis), Si-29 MAS NMR, and electron spin resonance (ESR) spectroscopic techniques. The XRD patterns indicated an increase in the d(100) spacing (from 36.77 to 40.88 Angstrom) with increasing Zr content. The samples showed a characteristic oxygen to zirconium charge-transfer band at 210 nm in the DRUV-vis spectra, consistent with monatomic dispersion of zirconium in Zr-MCM-41. Reduction of zirconium from the +4 to the +3 state was achieved by reaction with LiAlH4 at 298 K or with dry hydrogen at high temperatures. In samples A and B, Zr is located mostly in the pore walls, in substitutional locations (type I') and in C and D, it is present at the surface of the pores (type I ") in addition to the substitutional locations. Zr(III) ions, in type I' locations (g(1) = 1.876, g(2) = 1.943 and g(3) = 1.970), were resistant to reoxidation while those in type I " locations (g(parallel to) = 1.899 and g(perpendicular to) = 1.961) readily oxidized and formed Zr-IV(O-2(.-)) species. Interaction with H2O2 or gamma -ray irradiation also produced the superoxo radical species. Irradiation resulted in two types of superoxo radical species, type II' ions associated with surface Zr centers (g(1) = 2.0319, g(2) = 2.0093, and g(3) = 2.0024) and type II " ions associated with defect centers (g(1) = 2.0131, g(2) = 2.0098, and g(3) = 2.0045). The ESR results of Zr(III) ions are compared with those of the isoelectronic Ti(III) centers in silicalite structures.