Abstract: In this communication the synthesis and physical properties of the new ionic liquid 1-ethyl-3-methyl imidazolium decyl sulfate, are shown. The interest of this new salt lies in its tendency to jellify when it is exposed to the atmosphere by absorbing water from moisture up to about a 10% of its own mass; then it keeps the mass ±2% stable depending on the atmospheric humidity grade. The rigid-gel state appears from 15 ºC to 60 ºC. Below 15 ºC it crystallizes and above 60 ºC it liquefies. According to TGA measurements the compound is stable up to 300ºC. 1-ethyl-3-methyl imidazolium decyl sulfate was prepared in two steps. First the alkylation of 1-methylimidazole with diethyl sulfate afforded 1-ethyl-3-methylimidazolium ethyl sulfate. Then a later trans-esterification reaction allowed obtaining the desired 1-ethyl-3-methyl imidazolium decyl sulfate with high yield and purity. Its structure was confirmed by 1H, 13C NMR and IR spectroscopy as well as Mass Spectrometry. The FTIR spectrum of this new compound, and the temperature behavior of some physical properties such as electrical conductivity, viscosity, density and refractive index, will be presented.