The development of steam-stable materials suitable for sustainable CO2 capture in the postcombustion process is important for global CO2 reduction. In this study, covalently bonded polyethylenimine (PEI) modified multiwalled carbon nanotubes (CNT) were analyzed with multiple characterization methods, while the covalent amide bonds were identified using X-ray photoelectron spectroscopy. Analysis with multiple linear regression indicated that surface area and total number of functional groups installed were the dominant factors that contributed to covalent PEI loading. CO2 capture and stability of the material were evaluated under dry and steam conditions. Under steam conditions, the CO2 capture of the new materials improved by 14% compared to dry conditions, and the PEI loading and CNT surface chemistry remained stable. In contrast, CO2 capture and PEI loading for physisorbed PEI-CNT decreased for steam conditions compared to dry conditions. These results indicate that the synthesis of carbon-bonded amine materials with high stability and sustainability under humid conditions can be achieved.