Phenol, toluene and ethyl benzene hydroxylation were studied to investigate the potential benefits of semi-batch operation on aromatic hydroxylation reactions. Small crystallites of TS-1 and Al-free Ti-Beta were used as catalysts either in closed batch reactors or in a semi-batch system with the hydrogen peroxide added slowly over time. Water and methanol were used as solvents for all substrates and acetonitrile and solvent free systems were investigated additionally for toluene and ethylbenzene. It was seen that tar formation was higher when more H2O2 was present. For phenol hydroxylation, semi-batch operation significantly improved reaction performance with specific catalyst/solvent combinations. The best results were seen with Al-free Ti-Beta using the methanol solvent. This was attributed to increased formation of di(hydro)peroxo species in the pores of Al-free Ti-Beta, particularly in the presence of the methanol. In all cases, the lifetime of the catalyst was observed to increase when using semi-batch operation due to the decreased tar formation. Semi-batch operation also improved the catalyst lifetime for toluene hydroxylation as well as improving the reaction activity and selectivity in select cases. (C) 2011 Elsevier B.V. All rights reserved.