For this work we performed quantum mechanical (QM) and statistical mechanical (SM) calculations to generate the entropy of 13 aromatic compounds-naphthalene, 2 methylnaphthalene isomers, and 10 dimethyinaphthalene isomers-in the ideal gas state. Density functional theory (DFT) was used to calculate the equilibrium structure and perform a full normal-mode analysis. The DFT level of theory used in this paper is B3LYP/6-31G(d,p). DFT has also been used to determine barriers for the internal rotation contribution to the entropy. For four compounds for which experimental data are available, the calculated entropies have been compared to the experimental values. The calculated entropies match experiment very well, with the percentage errors close to the experimental uncertainty, less than 0.4 %. The equilibrium distribution of dimethylnaphthalene isomers in the mixture is predicted using the calculated entropies and energies from QM and SM calculations in the 300 K to 740 K temperature range.