The determination of the concentrations of ionic liquids (ILs) in water by UV spectroscopy was proposed. The effects of alkyl chain length, anion, cation, and C2 methylation of ILs were systematically investigated. The concentration detection limit ranges from 0.625 to 80 mg/L, and can be extended by changing the path length of the UV cuvette. In addition, the conjugated and inductive effects of ILs were analyzed by the maximum absorption wavelength λmax. Cations without conjugated effect (e.g., pyrrolidium, tetraammonium, and tetraphosphonium) show no UV spectra peak, while cations with conjugated effects (e.g., imidazolium, and pyridinium) present UV spectra peaks. Electron‐donating substitute groups (e.g., alkyl, C2 methylation) induce a larger value of maximum absorption wavelength λmax than the electron‐withdrawing substitute groups (e.g., amine, non‐C2 methylation). These findings are helpful to detect and quantify the concentration of ILs in water.