Room temperature ionic liquids of cyclic sulfonimide anions n cPFSI (ring size: n = 4-6) with the cations EMIm + (1-ethyl-3-methylimidazolium), BMIm + (1-butyl-3-methylimidazolium) and BMPL + (BMPL = 1-butyl-1-methylpyrrolidinium) have been synthesized. Their solid-state structures have been elucidated by single-crystal X-ray diffraction and their physicochemical properties (thermal behaviour and stability, dynamic viscosity and specific conductivity) have been assessed. In addition, the ion diffusion was studied by pulsed field gradient stimulated echo (PFGSTE) NMR spectroscopy. The decisive influence of the ring size of the cyclic sulfonimide anions on the physicochemical properties of the ILs has been revealed. All ILs show different properties compared to those of the non-cyclic TFSI anion. While these differences are especially distinct for ILs with the very rigid 6cPFSI anion, the 5-membered ring anion 5cPFSI was found to result in ILs with relatively similar properties. The difference between the properties of the TFSI anion and the cyclic sulfonimide anions has been rationalized by the rigidity (conformational lock) of the cyclic sulfonimide anions. The comparison of selected IL properties was augmented by MD simulations. These highlight the importance of π + -π + interactions between pairs of EMIm + cations in the liquid phase. The π + -π + interactions are evident for the solid state from the molecular structures of the EMIm + -ILs with the three cyclic imide anions determined by single-crystal X-ray diffraction. Ionic liquids with three cyclic perfluoroalkylsulfonylimide anions that are related to the bis(trifluoromethylsulfonyl)imide anion are described, which show the importance of conformational flexibility on IL properties.