In this research, the mono esters of glycerol have been synthesized by two methods :direct esterification at glycerol by phenylacetic acids in mediate of Amberlyst-15,10% mol and the other method base on transferring the acyl group from the phenylacetic anhydride to glycerol in mediate of acyl transfer agents and basic median of pyridine. The ester has been synthesized from anhydrides is more efficient than one from the carboxylic acid. The pKa of the Prepared esters have been determined and compared with the pKa of acids that used as starting materials.