A process for alkylation of an alkylatable aromatic compound to produce a monoalkylated aromatic compound, comprising the steps of: (a) providing at least one reaction zone having a water content with at least one alkylation catalyst having an activity and a selectivity for said monoalkylated benzene, said alkylation catalyst comprising a porous crystalline molecular sieve of a MCM-22 family material, said MCM-22 family material is characterized by having an X-ray diffraction pattern including d-spacing maxima at 12.4.+-.0.25, 3.57.+-.0.07 and 3.42.+-.0.07 Angstroms; (b) supplying the reaction zone with at least one alkylatable aromatic compound and at least one alkylating agent; (c) operating the reaction zone under suitable alkylation or transalkylation conditions, to produce at least one effluent which comprises a monoalkylated aromatic compound and a polyalkylated aromatic compound(s); (d) monitoring the amount of the monoalkylated aromatic compound or the amount of the polyalkylated aromatic compound(s) in the effluent; (e) adjusting the water content in the reaction zone to secure a desired amount of the monalkylated aromatic compound or the polyalkylated aromatic compound(s) in the effluent, the water content in the reaction zone being in a range from about 1 wppm to about 900 wppm; and wherein the polyalkylated aromatic compound(s) produced is reduced as compared to the reaction zone having a water content of about 0 wppm when the reaction zone is operated under equivalent conditions.