The emissions of Taylor cones of a wide range of ionic liquids hove been tested in vacuo in tin attempt to identify what physical properties favor the purely ionic regime (PIR). This regime is well known in the case of Taylor cones of liquid metals. It has been previously observed in conventional (capillary tube) electrospray sources at room temperature only for the room temperature molten salt (ionic liquid) EMI-BF4 (EMI = 1-ethyl-3- methylimidazoliuin). A large number of other ionic liquids and their mixtures have been studied here, most of which are unable to reach the purely ionic regime at room temperature. Based on these results and additional theoretical considerations we conclude that the purely ionic regime is favored not only by ionic liquids of high electrical conductivity, but also by those having relatively high surface tension.