Speed of sound measurements are currently of the utmost importance for the development of thermodynamic models of fluids. A need for accurate thermodynamic models was identified for siloxanes, because of their use as working fluid in organic Rankine cycle power systems and because of the scientific interest in non ideal and non classical gas dynamics. The initiative of designing and realizing a novel acoustic resonator capable of performing speed of sound measurements in dense vapors at temperatures up to 400 ∘ C stemmed from these considerations. Preliminary and unique experimental results are presented for siloxane D 6 (dodecamethylcyclohexasiloxane) for two isotherms at 346 ∘ C and 348 ∘ C and at pressures ranging form 5.5 bar to 6 bar. These measurements demonstrate for the first time the possibility of measuring the speed of sound of dense organic vapor at very high temperature. An initial comparison with speed of sound estimations provided by current thermodynamic models shows that the calculated values are approximately 6 % higher.