During thermal evolution, macromolecules in coal is continuously broken off to form small molecules due to the difference between internal bond energy varying with thermodynamic conditions, the small molecules cut off are interacted with solid organic particles immediately, and they are "trapped" by particles on the pore surface. The action between hydrocarbon molecules and pore surface occurs firstly in the micropore and transition pores in coal. One reason is that the smaller pores need less condensation pressure; the other is that molecule gravity field between adjacent pore walls overlap each other. The hydrocarbons besides consuming micropore can fill up meso- and macro-pores with the increasing hydrocarbon generation. The hydrocarbons reserved macropore are easy to desorbed. The adsorption experiment of C1 ～ C9 n ～ hydrocarbons in solid ammonium molybdothosphape proved that longer chain alkanes were desorbed slowly under indoor temperature. In fact, the extractable amounts in coal are larger than of bitumen "A" when pyridine was used as solvent. Similarly, the hydrocarbon filled in the micropores must partly contribute to the peak S2 in Rock-Eval experiment.
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