Quantum sieving of activated carbon fibers (ACFs) and their fluorides was observed for H-2 and D-2 adsorption at 20 K. Fluorination reduced the slit-shaped pore width of ACFs by 0.2 nm. The activated carbon fibers can act as highly efficient quantum sieves for H-2 and D-2, because the effective size of an H2 molecule is larger than that of a D-2 molecule due to the uncertainty principle and the molecular size difference between H2 and D-2 is significant in the micropore space. The D-2/H-2 selectivity of ACFs evaluated by ideal adsorption solution theory was larger than that of the fluorinated ACFs.