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The hydroxylation of phenol into catechol and hydroquinone with aqueous H₂O₂ (30%) and titanium-substituted molecular sieves (TS-1 and Al-free Ti-Beta) was investigated to understand the role of the zeolite structure, the crystal size, the external surface of the zeolite, and the nature of the solvent on the product selectivity. Comparing Al-free Ti-Beta and TS-1 samples with similar pore lengths, the activity as well as the ratio of catechol to hydroquinone was significantly higher for Al-free Ti-Beta, showing diffusional constraints for the conversion of phenol and geometric constraints for the formation of catechol. The diffusional constraints in the conversion of phenol were confirmed by using small crystallites of TS-1. The role of the external surface of TS-1 in the phenol hydroxylation was investigated by inertization of the external surface, using cycles of low-temperature chemical vapor deposition (CVD) of …