The alkylation of phenol with propylene has been studied over several H-ZSM-5s with different Si/Al ratios and Cs+-ion-exchanged H-ZSM-5s at temperature range 373-623 degreesC. Both O- and C-alkylation, which were closely dependent on the reaction temperature and acidity of the catalysts, were observed. O-alkylated compound is found to be formed preferably at temperature lower than 250 degreesC and over Cs+-ion-exchanged H-ZSM-5s. However, at higher temperature, only C-alkylation is observed. The acidic properties of the zeolites were characterized by solid-state P-31 MAS-NMR of the probe molecule trimethylphosphine oxide and NH3-TPD (temperature-programmed desorption) and it is suggested that in the case of C-alkylation, moderate acid sites are responsible for the formation of para-isopropylphenol, while ortho-isopropylphenol is favorable for weak acid sites.