In order to understand the behavior of COS in a ZnO desulfurization reactor, the reaction between ZnO and COS was studied in the presence of gases which compose a coal-derived gas. The behavior of COS in the reaction zone of a ZnO packed bed can be predicted as follows : H2S in coal-derived gas reacts more easily with ZnO than COS; most of COS is converted to H2S by catalytic hydrolysis and then reacts with ZnO, although a part of COS may react directly with ZnO; H-2 accelerates the conversion of COS to H2S; the water-gas shift reaction accelerates the reaction between ZnO and COS; and CO2 does not affect the reaction.