The acidity of a zeolite catalyst is reduced by calcination in an essentially water-free atmosphere at temperatures above 700.degree. C., preferably from 725.degree. to 800.degree. C., to reduce the alpha value to less than 10 percent of its original value. The low acidity catalysts produced in this way may be used for isomerizing alkyl di-substituted aromatic compounds, especially xylenes, to products containing higher proportions of the para-isomers.