Abstract Protic ionic liquids (PILs) are considered as potential solvents for CO2 capture due to their simple synthetic routes and unique properties. In this work, three low viscous PILs, tetramethylgunidinium imidazole ([TMGH][Im]), tetramethylgunidinium pyrrole ([TMGH][Pyrr]) and tetramethylgunidinium phenol ([TMGH][PhO]) were synthesized and the effect of anions, temperature, CO2 partial pressure and water content on CO2 absorption performance of PILs was also systematically studied. It was found that the PILs with larger basicity show higher CO2 absorption capacity, and [TMGH][Im] simultaneously shows relatively high absorption rate and CO2 absorption capacity of 0.154 g CO2/g IL at 40 °C, 1 bar. The addition of H2O has a positive effect on gravimetric absorption capacity of CO2 at the range of 0–20 wt% H2O, and the highest capacity of 0.186 g CO2/g IL was achieved as the water content was 7 wt%. In situ FTIR, 13C NMR and theoretical calculations verified that more stable bicarbonate are produced during CO2 absorption by [TMGH][Im]-H2O system. However, neat [TMGH][Im] can react with CO2 to form the reversible carbamate, leading to excellent recyclability after four absorption-desorption cycles. The results implied that neat [TMGH][Im] shows great potentials in CO2 absorption applications.