The syntheses of transparent colloidal solutions of extremely small titanium dioxide particles ( d < 3 nm) in water, ethanol, 2-propanol, and acetonitrile are presented. Quantum-size effects are observed during particle growth and at the final stages of synthesis. They are quantitatively interpreted by using a quantum mechanical model developed by Brus. The particles prepared in aqueous solution possess the anatase structure and consist of about 200 TiO, molecules at their final growth stage. The colloidal particles can be isolated from solution as white powders that are soluble in water and ethanol with no apparent change in their properties. In organic solvents the quantum-size TiOz particles appear to form with rutile structure. Excess negative charge on the particles resulting either from deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers causes a blue shift in the electronic absorption spectrum, which is explained by an electrostatic model. Electrons can be trapped in the solid as a Ti3+ species, which has a characteristic visible absorption spectrum. As much as 10% of the available Ti4+ ions can be reduced photochemically in the solid with a quantum yield of 3%. Molecular oxygen reoxidizes the Ti3+ centers, leading to detectable amounts of surface-bound peroxides. The pH of zero point of charge (pH,) of the aqueous colloidal suspension has been determined to be 5.1 f 0.2. The acid-catalyzed dissolution of the aqueous colloid yielding Ti(1V) oligomers has been studied, and an activation energy E, = 58 f 4 kJ/mol has been measured for this reaction. The photocatalytic activity of the small Ti02 particles is demonstrated.