This article presents the development of technical equations of state for four siloxanes using the 12-parameter Span-Wagner functional form. Siloxanes are used as heat transfer fluids and working media in energy conversion applications. The investigated fluids are two linear dimethylsiloxanes, namely MM (hexamethyldisiloxane, C6H18OSi2) and MD4M (tetradecamethylhexasiloxane, C14H42O5Si6), and two cyclic dimethylsiloxanes, namely D-4 (octamethylcyclotetrasiloxane, C8H24O4Si4) and D-5 (decamethylcyclopentasiloxane, C10H30O5Si5). Available measured properties are critically evaluated and selected for the optimization of the equation of state (EoS) parameters. Due to the insufficient number of experimental values, several other properties are estimated with the most accurate ad hoc methods. These estimates are included in the optimization of the equation of state parameters. Moreover, experimental saturated liquid density and vapor pressure data are correlated with the equations proposed by Daubert and Wagner-Ambrose, respectively, to provide short, simple, and accurate equations for the computation of these proper-ties. The performance of the obtained equations of state is assessed by comparison with experimental data and also with estimates obtained with the Peng-Robinson cubic EoS with the modification proposed by Stryjek and Vera. This equation was adopted in previous technical studies. The improvements obtained with the newly developed EoS's are significant. Exemplary state diagrams are also reported as a demonstration of the consistency of the obtained thermodynamic models. Sound speed measurements in the vapor phase are planned for the near future and results will be incorporated in future improvements of the newly developed thermodynamic models. (c) 2006 Elsevier B.V. All rights reserved.