A two-pronged approach has been used to obtain accurate ideal-gas heat capacities of cyclic and linear dimethylsiloxanes that are useful for thermodynamic modeling of several processes involving these compounds. Acoustic resonance measurements were made on gas-phase octamethylcyclotetrasiloxane (D-4, [(CH3)(2)-Si-O](4)) and decamethylcyclopentasiloxane (D-5, [(CH3)(2)-Si-O](5)) over the temperature range 450-510K. These new data, along with previously published molecular vibrational frequency data for hexamethyldisiloxane (MM, [(CH3)(3)-Si-O-1/2](2)), were used to develop an appropriate frequency scaling factor that can be used with ab initio frequency calculations to produce reliable ideal-gas heat capacities as a function of temperature. Ideal-gas heat capacities for both cyclic [(CH3)(2)-Si-O](n) (with 3 <= n <= 8) and linear (CH3)(3)-Si-O-[(CH3)(2)-Si-O](n)-Si-(CH3)(3) (with 0 <= n <= 5) siloxanes over a wide range of temperatures were determined with the ab initio, method. (c) 2007 Elsevier B.V. All rights reserved.