Twelve N-SO3H functionalised ammonium and imidazolium based Brønsted acidic ionic liquids were synthesised by varying four anions (Cl-, CF3COO-, BF4- and OTf-). They were characterised using various spectroscopic techniques. A detailed study on Brønsted acidity, thermal stability, conductivity, density, polarity, and electrochemical window of the ionic liquids (ILs) was carried out to analyse the effects of different cations and anions. The conductivity of these ILs in molecular solvents (MeOH, MeCN and acetone) were measured at a particular mole fraction (XIL = 0.001) of the ILs and the results were correlated with Kamlet-Taft solvatochromic parameters (ETN, α, β) of the solvents. Their Brønsted acidity order was determined using the Hammett plots. The polar nature of synthesized ionic liquids was studied by taking fluorescence emission spectrum of pyrene as a probe molecule in two illustrative 2-ethyl-1,3-disulfoimidazolium ionic liquids. The electrochemical stabilities of twelve ILs in the same molecular solvents were determined by Cyclic Voltammetry. The results revealed highest electrochemical stability window for the imidazolium ILs in acetone. Among the four types of anions, the ionic liquids of CF3COO- and Cl- anions were found to be thermally stable up to 250–260 °C as compared to the ionic liquids of BF4- and OTf- anions.