A facile method to determine the concentration of Cu2+ ions in aqueous solution was developed by using ionic liquids (ILs) as a concentrating reagent. Four hydrophobic ILs with the bis(trifluoromethanesulfonyl) imide counter anion were tested for their abilities to measure Cu2+ ion concentration. Results showed that 1-allyl-3-butylimidazolium bis(trifluoromethanesulfonyl) imide (ABIM) was good at concentrating Cu2+ ions in the form of [(TMPyP) Cu](4+), and thus the ABIM ILs only extracted [(TMPyP) Cu](4+) species from aqueous solution, whereas others showed no selectivity. In UV-vis absorption spectra, the Soret band absorbance of [(TMPyP) Cu](4+) in ABIM ILs was 17 times higher than that in water. Moreover, the band intensity increased linearly with increasing Cu2+ ion concentration. These results indicated that ABIM IL-TMPyPH2 (TMPyPH2: meso-tetrakis(N-methyl-pyridinium-4-yl) porphyrin) was an effective concentrator for Cu2+ ions. Interference studies showed that other metals (Cd2+, Zn2+, Fe3+, Mn2+, and Ni2+) did not interfere significantly with the determination of Cu2+ ion concentration using this method.