In-situ C-13 MAS NMR techniques were used to investigate the effects of different solvent systems on the formation of phenylacetalclehyde (PADH) over TS-1 zeolite, Protic solvents such as water and methanol provide acidic centers during the reaction. These acidic species catalyzed the transformation of the intermediates to PADH. Whereas in the presence of aprotic solvents such as acetone, the precursor of PADH remains stable on the framework of TS-1 and accordingly there is no PADH formation. (C) 2009 Elsevier Inc. All rights reserved.