A catalyst based on a mesoporous phenol-formaldehyde polymer (MPF) as an organic support modified with the IMHSO4 ionic liquid has been synthesized. The catalytic activity of the sample has been studied in the alkylation of aromatic compounds with octene-1. It has been found that the use of the catalyst in phenol alkylation leads to the formation of both alkylphenols (C-alkylates) and alkyl phenyl ethers (O-alkylates) with a total yield of up to 60%. In the case of alkylation of benzene and benzene derivatives, significant conversion values (45-50%) are achieved only for toluene and anisole.