The temperature variation of density, vapour pressure, and surface tension of the liquids have been measured, and the molecular volume, latent heat of vaporisation, and Kleeman-Macleod constants have been determined. The invariability of the latter constants over a wide range of temperature shows that the compounds behave as normal liquids.The solid-liquid equilibria of binary systems of the compounds have been examined, and it is shown that the type of equilibrium may be related t o the difference between the parachors of the components.In an examination of the isosterism of the CH group with the atoms of the elements of Group VB, i t is concluded that factors other than the weight of the groups are involved in isomorphism. The equilibria between tetraphenyltin and each of the triphenyl compounds have been determined, and earlier measurements have been corrected.No systematic study of the physical properties of liquid triphenyl compounds of the elements of Group VB has hitherto been made, although a few measurements of the density and surface tension over limited ranges of temperature have been carried out by Pascal (Compt. rend., 1913(Compt. rend., , 156, 1905 and by Walden and Swinne (2. physikal. Chem., 1912, 79, 713), respectively. In the present work it is found that the variation of density with temperature is normal for a range of 200' above the m. p., but triphenylbismuthine begins to decompose a t 180°. It will be evident from the data in Table I that Pascal's values of the molecular volumes, VsOo, of the solid are less regular than the estimated molecular volumes, V,, of the liquid a t the boiling point. If Kopp's values (Annalen, 1855, 96, 153) are taken for the atomic volumes of carbon (11.0) and hydrogen (5.5), the approximate atomic volumes of the elements of Group VB in this type of compound are : N, 16.5 ; P, 29.5; As, 34.5 ; Sb, 40.5.The atomic parachors given by Sugden (op. cit.) are : N, 12.5; P, 37.7 ; As, 50.1 ; Sb, 66.0; Bi, 80.