A large series of ionic liquids (ILs) based on the weakly coordinating alkoxyaluminate [Al(hfip)4]- (hfip: hexafluoroisopropoxy) with classical as well as functionalized cations were prepared, and their principal physical properties determined. Melting points are between 0 ([C 4MMIM][Al(hfip)4]) and 69 °C ([C3MPip] [Al(hfip)4]); three qualify as room-temperature ILs (RTILs). Crystal structures for six ILs were determined; their structural parameters and anion-cation contacts are compared here with known ILs, with a special focus on their influence on physical properties. Moreover, the biodegradability of the compounds was investigated by using the closed-bottle and the manometric respirometry test. Temperature-dependent viscosities and conductivities were measured between 0 and 80 °C, and described by either the Vogel-Fulcher-Tammann (VFT) or the Arrhenius equations. Moreover, conductivities and viscosities were investigated in the context of the molecular volume, Vm. Physical property-Vm correlations were carried out for various temperatures, and the temperature dependence of the molecular volume was analyzed by using crystal structure data and DFT calculations. The IL ionicity was investigated by Walden plots; according to this analysis, [Al(hfip)4]- ILs may be classified as "very good to good ILs" while [C2MIM][Al(hfip)4] is a better IL than [C2MIM][NTf2]. The dielectric constants of ten [Al(hfip)4]- ILs were determined, and are unexpectedly high (εr=11.5 to 16.8). This could be rationalized by considering additional calculated dipole moments of the structures frozen in the solid state by DFT. The determination of hydrogen gas solubility in [Al(hfip)4]- RTILs by high-pressure NMR spectroscopy revealed very high hydrogen solubilities at 25 °C and 1 atm. These results indicate the significant potential of this class of ILs in manifold applications. New RTILs: A series of ionic liquids (ILs) based on the weakly coordinating anion [Al(hfip)4]- (see figure) has been synthesized and thoroughly characterized. The ILs were found to have widely interesting properties that suggest potential for these ILs in many applications (hfip=hexafluoroisopropoxy). © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.