The standard chemical thermodynamic properties of the alkylcyclopentane isomer groups have been calculated through C9H18in the ideal gas phase from 298.15 to 1000 K, and the properties of the alkylcyclohexane isomer groups have been calculated through C10H20. The properties of individual species for which literature data are not available have been estimated using the Benson method. The increments per carbon atom in the isomer group properties have been calculated to determine the extent to which extrapolations may be made to higher carbon numbers. Since alkylcyclopentanes and alkylcyclohexanes of the same carbon number are isomers, the chemical thermodynamic properties of these combined isomer groups have also been calculated. Values ofC○P,S○, &Dgr;fH○, and &Dgr;fG○are given for the individual cyclopentane species through C9H18and for the individual cyclohexane species through C10H20for a standard state pressure of 1 bar.