The adsorption of H2O vapor on biomass char particles is gravimetrically measured in a temperature range of 298.15 K to 323.15 K. The results are correlated to results of a comprehensive structural and chemical char analysis using N2 and CO2 physisorption measurements with corresponding 2D-NLDFT models and temperature-programmed desorption measurements (TPD) for the detection of oxygen-containing functional groups (OFG). The adsorption isotherms of a highly porous and unfunctionalized model char show a distinct type V shape for hydrophobic, microporous materials, which is in accordance with its structural and chemical properties. In contrast, results for a highly functionalized model char show type II isotherm characteristics with high adsorption capacity at low H2O concentrations. The adsorption behavior of a beechwood char with a conversion history aligns more closely with that of the unfunctionalized model char, exhibiting differences that correlate with its less pronounced pore structure and higher proportion of OFG. Molecular dynamics (MD) simulations of ideal slit pores were conducted to confirm distinct effects and tendencies found in the H2O adsorption measurements with respect to outstanding char properties. The simulations confirm a strong binding tendency of H2O molecules to OFG, especially for comparably low H2O densities as well as a contribution of the confinement effect in small pores to the overall adsorption capacity.