A process for alkylation or transalkylation of an alkylatable aromatic compound having reactive impurities with an alkylating agent to produce a monoalkylated aromatic compound, comprising the steps of contacting at least a portion of said alkylatable aromatic compounds and said alkylating agent with a first molecular sieve catalyst in a guard bed under suitable conditions to remove said reactive impurities and form a first effluent comprising monoalkylated aromatic compound, unreacted alkylatable aromatic compounds and unreacted alkylating agent; contacting said first effluent with a second molecular sieve catalyst different from said first molecular sieve catalyst in said reaction zone under suitable alkylation or transalkylation conditions to produce additional said monoalkylated aromatic compounds; and maintaining said water content from about 1 wppm to about 10 wt. % based on the combined weight of said alkylatable aromatic compound and said alkylating agent in said reaction zone for the majority of the on-oil time.