Previously unavailable high-pressure solubility data of hydrogen in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide has been measured experimentally up to temperatures and pressures of 450 K and 15 MPa, respectively. In contrast to CO2 solubility, H2 tends to dissolve better in the ionic liquid at higher temperatures. This inverse temperature effect has been studied from a thermodynamic perspective and the underlying reason for this effect is explained. It is shown that the negative P-T slope is not limited to this particular binary mixture, but is the typical behavior in most, if not all, H2 + ionic liquid systems. However, there is a certain range of temperatures, pressures, and concentrations in which this phenomenon occurs. By predicting the Scott-van Konynenburg phase diagram for systems of H2 + ionic liquids to be of type III, it is shown how and why the solubility increases with temperature in some regions, but decreases in others. (c) 2012 American Institute of Chemical Engineers AIChE J, 2012