The acidity and surface properties of alumina modified with sulfate (Al-S) and phosphate (Al-P) were characterized by IR, NH₃ TPD and 1-butene isomerization reaction. The Point of Zero Charge (PZC) of modified alumina and the amount of Mo anion adsorbed on modified alumina were measured. The PZC of modified alumina and the amount of Mo anion adsorbed on modified alumina decreased with increasing modifier content. Non-linear relationship between the PZC and Mo uptake in modified alumina was observed in Al-S and Al-P. The bonding mode of sulfate on alumina changed with increasing sulfate content. The Bronsted acid site appeared in alumina modified with sulfate of 10 wr%.