The effect of post-treatment upon the H2O adsorption performance of biomass-based carbons was studied under post-combustion CO2 capture conditions. Oxygen surface functionalities were partially replaced through heat treatment, acid washing, and wet impregnation with amines. The surface chemistry of the final carbon is strongly affected by the type of post-treatment: acid treatment introduces a greater amount of oxygen whereas it is substantially reduced after thermal treatment. The porous texture of the carbons is also influenced by post-treatment: the wider pore volume is somewhat reduced, while narrow microporosity remains unaltered only after acid treatment. Despite heat treatment leading to a reduction in the number of oxygen surface groups, water vapor adsorption was enhanced in the higher pressure range. On the other hand acid treatment and wet impregnation with amines reduce the total water vapor uptake thus being more suitable for post-combustion CO2 capture applications. ; This work was carried out with financial support from the HiPerCap Project of the European Union 7th Framework Programme FP7 (2007–2013; Grant Agreement Number: 60855) and from the Gobierno del Principado de Asturias (PCTI-GRUPIN14-079). N.Q. acknowledges a fellowship from the Gobierno del Principado de Asturias (Programa Severo Ochoa). ; Peer reviewed