Substitution of methyl groups onto the aromatic ring determines the SOA formation from the aromatic hydrocarbon precursor. This study links the number of methyl groups on the aromatic ring to SOA formation from aromatic hydrocarbons photooxidation under low-NO_x conditions (HC / NO > 10 ppbC : ppb). Aromatics are determined to be less oxidized per mass/carbon as the number of methyl groups on aromatic ring increases based on SOA yield, SOA chemical composition and SOA physical characteristics.