The present invention provides a radiation-sensitive resin composition that contains a polymer having a structural unit that includes an acid-labile group; and an acid generator, wherein the acid generator includes a compound including a sulfonate anion having SO.sub.3.sup., wherein a hydrogen atom or an electron-donating group bonds to an carbon atom with respect to SO.sub.3.sup., and an electron-withdrawing group bonds to a carbon atom with respect to SO.sub.3.sup.; and a radiation-degradable onium cation. The compound preferably has a group represented by the following formula (1-1) or (1-2). In the following formulae (1-1) and (1-2), R.sup.1 and R.sup.2 each independently represent a hydrogen atom or a monovalent electron-donating group. R.sup.3 represent a monovalent electron-withdrawing group. R.sup.4 represents a hydrogen atom or a monovalent hydrocarbon group.

##STR00001##

An onium salt monomer consists of a sulfonic acid anion containing a polymerizable group and being free of iodine and a triarylsulfonium cation containing iodine and fluorine. A resist composition comprising a polymer comprising repeat units derived from the monomer exhibits a high solvent solubility, high sensitivity and high contrast and forms a small-size pattern with satisfactory exposure latitude, edge roughness, dimension uniformity and depth of focus when processed by photolithography using high-energy radiation, as well as collapse resistance.

An onium salt monomer consists of a sulfonic acid anion containing a polymerizable group and being free of iodine and a triarylsulfonium cation containing iodine and fluorine. A resist composition comprising a polymer comprising repeat units derived from the monomer exhibits a high solvent solubility, high sensitivity and high contrast and forms a small-size pattern with satisfactory exposure latitude, edge roughness, dimension uniformity and depth of focus when processed by photolithography using high-energy radiation, as well as collapse resistance.