Zirconium-containing medium-pore, Al-free molecular sieves (Si/Zr molar ratios > 50) with MFI structure have been synthesized both in alkaline and HF/CH3NH2 medium. The samples synthesized in HF/CH3NH2 medium exhibit a high crystallinity, bigger particle size, lower surface area and lower catalytic activity in the hydroxylation of phenol as compared to those synthesized in alkaline medium. An expansion in unit cell volume (XRD), an absorption at 965 cm(-1) (FTIR) and at 212 nm (UV-Visible) as well as the catalytic activity data indicate that upto 0.6 zirconium atom per unit cell. (Si/Zr = 158) could be substituted isomorphously within the silicalite-1 framework. The IR study of adsorbed pyridine has shown that strong Lewis acid and weak Bronsted acid sites exist on all zirconium silicate samples, synthesized in alkaline medium. The relative stability of cluster models representing various possible lattice structures and the ordering of molecular orbitals derived from extended Huckel molecular orbital calculations are discussed to understand the nature of active sites in the zirconium silicates.