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In the removal of volatile organic compounds (VOCs) with nonthermal plasma (NTP), the additive effect of water on VOC decomposition is an important issue to be solved since water is contained in VOC exhausts. Although there have been disputed discussions on the additive effect of water on the decomposition of VOCs in NTP, a unified mechanism has not been established yet. Therefore, the additive effect of water on the NTP decomposition of dichloromethane, toluene, and methanol was studied in the range of water content from 0.0% to 2.0% at 298 K and 373 K. Consequently, no rate-promoting effect of water has been observed in the conversion of dichloromethane, toluene, or methanol from 0.5% to 2.0% of water content, and water addition increased CO2 selectivity and improved carbon balance to different degrees, depending on the chemical structures of these VOCs at 298 K. These findings suggest that the precursors of CO and CO2 are different from each other, that their selectivities are affected by VOC chemical structure and reaction conditions, and that the direct enhancement of the chemical interaction between nonvolatile reaction intermediates and water promoted COx formation. Moreover, it has been shown that the VOC chemical structure affects the synergistic effects of water addition and temperature increase on CO2 selectivities.