The chemical thermodynamic properties of alkylbenzene isomer groups from C8H10to C9H12in 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 C10H14to C12H18have been calculated using Benson group values. For isomer group properties, increments 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 ofC○P,S○, &Dgr;fH○, and &Dgr;fG○are given for all species from C6H6to C12H18in joules for a standard state pressure of 1 bar.