In alkylating an alkylatable aromatic material to obtain a desired position isomer an alkylating agent is fed with an intermediate alkylatable material to a first reaction zone wherein alkylation occurs and the effluent is separated in a multiple-stage separation zone into the desired position isomer which is recovered and undesired isomers and unreacted material which are fed to a second reaction zone together with fresh feed and recycle alkylatable aromatic material, therein subjected to transalkylation and/or isomerization and the effluent is separated into unreacted alkylatable material which is recycled to the second zone and heavier material which is passed as the intermediate material to the first zone. In the preparation <PICT:0823891/IV (b)/1> of p-di-isopropylbenzene, benzene is fed by line 1 to reactor 17 together with recycle aromatic compounds from line 16. A mixture of benzene and mono- and poly-isopropyl benzenes is removed by line 19 to the fractionator 18 from which benzene is recycled by line 21 to line 1 and heavier material passes by line 20, is mixed with a propane-propylene mixture from line 3 and is introduced into the alkylation reactor 2. In fractionator 5 propane and unreacted propylene are removed by line 6; in fractionator 8 material boiling below the di-isopropylbenzenes is removed by line 9 and recycled to reactor 17; in fractionator 11 material boiling above the di-isopropylbenzenes is removed by line 13 and the di-isopropylbenzenes pass to separator 14 from which the para-isomer is removed by line 15 while the ortho- and metaisomers are recycled by line 16 to zone 17. Suitable aromatic feed materials are benzene, toluene, xylenes, naphthalene, methylnaphthalenes, a ,b -dimethylnaphthalene, anthracene, phenanthrene, chrysene, pyrene, triphenylene, diphenyl, phenol, anisole, hydroxyanisole and hydroxynaphthalenes. Suitable alkylating agents are ethylene, propylene, butylene, isobutylene, amylene, ethane-ethylene, propanepropylene, butane-butylene, methanol, ethanol, propanol, butanol, methyl-, ethyl-, propyl- or butyl-chloride, methyl-, ethyl- or propyl-bromide, dimethyl -, diethyl - or dipropyl - ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, methyl-, ethyl- or propyl-mercaptan, and dimethyl-, diethyl- or dipropyl-sulphide. Suitable catalysts in each of the reaction zones are the alkylation catalysts such as hydrogen fluoride, sulphuric acid, phosphoric acid, "solid" phosphoric acid, hydrogen fluoride-boron trifluoride, Friedel-Crafts catalysts such as aluminium chloride, zinc chloride, bismuth chloride, ferric chloride, titanium chloride, boron fluoride and aluminium chloridehydrogen chloride and inorganic oxides such as silica-alumina, silica-zirconia, alumina-zirconia, silica-alumina-zirconia and acid treated clays. Pressures of 1 to 100 atmospheres may be used, preferably at least one reactant being maintained in the liquid phase and temperatures of -40 DEG to 400 DEG C. depending on the catalyst and alkylating agent used. The position isomers may be separated by standard methods such as fractional crystallization, fractional distillation, solvent extraction, selective adsorption, fractional precipitation, chemical separation based on differences in reactivity or in rates of forming derivatives, extractive distillation and thermal p diffusion. A single isomer need not be separated, a mixture of isomers can be produced, e.g. a mixture of higher melting dimethylnaphthalene can be produced to the exclusion of lower melting isomers.