Following previous workers, the surface tension of room temperature ionic liquids (RTILs) is shown to depend reciprocally on the surface area of the void space in the RTILs. The void space in liquids in general and in RTILs in particular can be expressed in several manners. The fraction of free space in RTILs can be estimated well and most simply from their actual molar volumes and the ionic volumes of the constituting ions. The temperature dependence of the surface tension is explored and shown to be essentially independent on the natures of the RTILs. (C) 2017 Elsevier B.V. All rights reserved.