The relationship between petrographic composition and swelling ratio in the 200-300 degrees C range for five subbituminous to high-volatile bituminous coals has been investigated The amount of fluorescing huminite / vitrinite plus the total liptinite macerals (i.e., ＇＇reactive＇＇ macerals) of the coal is the determining factor of coal swelling behavior There is an increase in swelling ratio with increasing coal rank as expressed by mean huminite / vitrinite reflectance. For coals of the same or very similar rank the swelling characteristics were directly related to the amount of fluorescing huminite / vitrinite in the coal. It is also observed that the fluorescence of vitrinite in the three higher rank coals showing better swelling ratios is related to the presence of fluorophases and labile hydrogen in the vitrinite matrix The two lower rank coals that exhibit poor swelling characteristics lack any visible fluorescence in the huminite matrix.