Effects of water vapor during Ar-GCIB irradiations on poly-methyl methacrylate (PMMA) were investigated. Because of chemical reaction enhancement between PMMA and water vapor adsorbed on PMMA by GCIB irradiations, etching depth of PMMA with water vapor became almost twice as deep as that without water. XPS showed that there was small change in chemical bond after irradiation of Ar-GCIB with water vapor. In addition, large Ar-GCIB irradiation (i.e., low energy/atom) in water vapor also helped to reduce irradiation damage on PMMA. (C) 2014 Elsevier B.V. All rights reserved.