The potentialities in the use of biochars prepared by steam-assisted slow pyrolysis as adsorbents of gases of strategic interest (N2, CO2, and CH4) and their mixtures were explored. The biochars prepared from Populus nigra wood and cellulose fibers exhibited a narrow microporosity, with average pore sizes ranging between 0.55 and 0.6 nm. The micropore volume increased with the pyrolysis temperature, allowing CO2 and CH4 uptakes at room temperature between 1.5 and 2.5 mmol/g and between 0.1 and 0.5 mmol/g, respectively. These values are in line with those from the literature on biomass-derived carbon-based materials, exhibiting much higher porous features than those reported herein. As for the separation of CO2/N2 and CO2/CH4 gas mixtures, data showed that the prepared biochars exhibited good selectivities for CO2 over both N2 and CH4: between ca. 34 and 119 for a CO2/N2 mixture in typical post-combustion conditions (15:85, v/v) and between 14 and 34 for a CO2/CH4 mixture typical of natural gas upgrading (30:70, v/v).