Abstract: A process for producing 99.5+ wt. % para-xylene by selective alkylation has been discovered. A mixure of isomers of para-xylene having at least about 90.0 wt. % purity may be reacted with an alkylating agent having at least one tertiary butyl moiety in the presence of an acidic catalyst under conditions where the catalyst is in good molecular contact with the non-para-xylene isomers. The catalyst may be a high surface area acidic heterogeneous catalyst such as a proton-exchanged zeolite. Upon distillation, para-xylene is recovered in 99.5 wt. % purity or higher. The selective alkylation gives little or no alkylation of the para-xylene, the most predominant species, and very little or no isomerization to the other, non-desirable isomers.