In this paper, we discuss the effects of arsenic implantation on methyl silsesquioxane (MSQ) film properties, in particular the resistance of MSQ films to O-2 plasma damage. Fourier-transform infrared spectroscopy (FTIR), secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) were used to study structural and compositional changes in As-implanted MSQ samples with and without exposure to O-2 plasma. Ellipsometry was used to measure the refractive index and thickness of the As-implanted samples. C-V measurements were performed to obtain dielectric constants of the samples. Our results show that after As implantation, carbon bonds in the MSQ films are broken, and hence change the structure of the MSQ. After bond breaking, carbon remains in the MSQ films as shown in the SIMS profiling, and the carbon atoms/ions are in an amorphous or unbonded state instead of bonding with Si, as observed from the XPS results. It is proposed that these carbon atoms/ions are trapped in a dense O-Si-O network and no longer react with oxygen. Thus, the structurally changed MSQ material has an increased resistance to O-2 plasma; however, these films also show an increase in the dielectric constant.