Carbon molecular sieve (CMS) membranes were prepared on the outer surface of porous ceramic tubes using a lignin-based material, organosolvelignin, and a phenol resin, Bellpeal-S899, as precursors. Higher pyrolysis temperature resulted in higher hydrogen selectivity over oxygen or other larger gases, but hydrogen permeance decreased significantly. On the contrary, addition of a small amount of iron(III) acetate basic to the precursors improved the hydrogen selectivity but maintained the hydrogen permeance. Addition of iron compound to the precursor strongly reduced the permeances of nitrogen and methane through the resulting CMS membranes. Distribution of iron in the carbon matrix was evaluated by transmission electron microscopy energy dispersive X-ray spectroscopy (TEM-EDS) and electron probe microanalyzer (EPMA). The influence of aging on the membrane permeance was also studied. The results suggest the possibility to tune the size of ultra-micro-pores in CMS membranes using additives.