Mixtures of C12 bicyclic aromatic hydrocarbons containing dimethylnaphthalenes are difficult to fractionate by conventional techniques such as distillation or crystallization. However, by dissolving a tetrahalophthalic anhydride in such a mixture at an elevated temperature, followed by subsequent cooling, a solid complex of certain hydrocarbons and the anhydride is formed. Separation of the solid complex from the cooled mixture and its subsequent decomposition results in a product that is substantially richer in those dimethylnaphthalenes which are preferentially complexed. Upon further processing, these dimethylnaphthalenes have utility in the production of dyes.