Trimethylbenzenes (TMBs) and C9 aromatic hydrocarbon solvents are structurally similar and have similar toxicity. Based on a review of the entire TMB and C9 aromatic hydrocarbon solvents toxicology database, oral and inhalation studies were identified to serve as the basis for a Reference dose (RfD) and Reference concentrations (RfC). The RfD and RfC were derived using standard USEPA methods and assumptions. The RfD was calculated to be 0.4 mg/kg/day using a 90-day oral study that resulted in a NOAEL of 600 mg/kg/day, based on a lack of adverse effects at the highest dose level (reversible effects such as increased serum phosphorus levels and liver and kidney weights), along with a total uncertainty factor of 1000. For the RfC, three studies were considered based on different study designs and toxicological endpoints, including neurotoxicity, systemic toxicity, and potential developmental and reproductive toxicity. For all three studies, as the calculated RfCs were consistent (3–4 mg/m3), the most conservative RfC, 3 mg/m3, was selected. The C9 aromatic hydrocarbon solvents referred to herein are based on chemistries assessed as part of the TSCA Section 4 Test Rule. These solvents contain primarily ethyl toluene and tri-methyl benzene isomers, but the specific compositions can vary based on feedstock and manufacturing process, thus, it is important to consider the composition of any specific solvent to assess similarity to that assessed in the TSCA Section 4 Test Rule program.