The physicochemical and catalytic properties of phosphorus-promoted H-ultrasils were examined in the disproportionation of ethylbenzene. It was shown that the chemical modification of H-ultrasil with ammonium hydrogen phosphate leads to the redistribution of acid sites and a decrease in the sorption capacity of the zeolite. As a result, the ratio between Lewis and Bronsted acid sites in the zeolite increases in a more pronounced manner and zeolite channels narrow, thereby enhancing the paraselectivity for isomers.