The mechanism of SO2 micropore filling in slit-shaped micropores was examined by direct calorimetry and potential calculation, and the effect of the pore width on the SO2 micropore filling mechanism is discussed. Four kinds of activated carbon fibers of different pore widths have been used, and both granular activated carbon and microporous Boehmite aggregates were used for comparison. The differential energy of SO2 adsorption (q(d)) on ACFs varied with change of the average pore width. An increase of the micropore width leads to a decrease of q(d) in SO2 micropore filling by decreasing the enhanced micropore field. Molecular potential calculations gave good agreement with the experimental adsorption energy when the image potential due to the SO2 dipole moment was considered. The image potential contributes 25-32% to the total molecular potential, indicating its importance in micropore filling of dipolar SO2.