A previous analysis of the pressure-volume-temperature behaviour showed that the linear dimethylsiloxanes conform to a phenomenological principle of corresponding states resembling but not identical to that of the normal alkanes, A similar analysis for tetramethylsilane-regarded as the first member of the dimethylsiloxane homologous series : two cyclic dimethylsiloxanes, octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane, and two polydimethylsiloxanes, one linear and one cyclic-confirms that they conform to a similar principle, The results are used to predict with fair success the excess volumes of mixing of a series of six related mixtures of cyclic substances-cyclohexane, cyclooctane and cyclodecane + either octamethylcyclotetrasiloxane or decamethylcyclopentasiloxane-and two linear + cyclic substance mixtures-either n-hexane or decamethyltetrasiloxane + octamethylcyclotetrasiloxane.