Protic ionic liquids have been regarded as promising materials to capture CO2, because they can be easily synthesized with an attractive capacity. In this work, we studied the CO2 absorption mechanism by protic ionic liquids (ILs) composed of diamino protic cations and azolide anions. Results of 1H nuclear magnetic resonance (NMR), 13C NMR, 2-D NMR and fourier-transform infrared (FTIR) spectroscopy tests indicated that CO2 reacted with the cations rather than with the anions. The possible reaction pathway between CO2 and azolide-based protic ILs is proposed, in which CO2 reacts with the primary amine group generated from the deprotonation of the cation by the azolide anion.