The chemical thermodynamic properties of alkylnaphthalene isomer groups for C10H8 and C11H10 in the ideal gas phase have been calculated from 298.15 to 1000 K from tables of Stull, Westrum, and Sinke. In the absence of literature data on all isomers of higher isomer groups, the properties of isomers of C12H12 to C14H16 have been calculated using Benson group values. A new Benson group value for the 1,8‐dimethyl steric hindrance has been calculated from recent experimental data. The increments in isomer group properties per carbon atom have been calculated to show the extent to which thermodynamic properties of higher isomer groups may be obtained by linear extrapolation. Equilibrium mole fractions within isomer groups have been calculated for the ideal gas state from 298.15 to 1000 K. Values of C○p, S°, ΔfH°, and ΔfG° are given for all species from C10H8 to C14H16 with energy units of joules for a standard state pressure of 1 bar.