Chemically amplified positive resist compositions comprising a polymer obtained by copolymerizing a silicon-containing monomer with a polar monomer having a value of LogP or cLogP of up to 0.6 and optionally hydroxystyrene, a photoacid generator and an organic solvent are sensitive to high-energy radiation and have a high sensitivity and resolution at a wavelength of less than 300 nm and improved resistance to oxygen plasma etching.