The effect of the microstructure of coconut-shell-based activated carbons, and their treatment with chemicals, on hydrogen adsorption capacity was studied. Active carbons from coconut shells using physical activation could be improved for hydrogen adsorption by treatment with HF or NH3•H2O. Treatment with HNO3 had no obvious effect and the treatment with H3PO4 lowered the hydrogen adsorption capacity considerably. All of the effects could be accounted for by changes in the specific surface area, pore size distribution, and surface chemistry of the active carbons. The specific surface area of carbons was the dominant factor for hydrogen adsorption no matter what kind of treatment was performed.