A resist underlayer film composition is excellent in resistance to a basic hydrogen peroxide aqueous solution in gap-filling and planarization characteristics having a dry etching characteristic; a patterning process and method for forming a resist underlayer film, wherein the resist underlayer film composition is used for a multilayer resist method, the composition comprising: (a1) one, or two or more, of a compound represented by following general formula (x); and (b) an organic solvent, wherein n01 represents an integer of 1 to 10; when n01 is 2, w represents a sulfinyl group, a sulfonyl group, an ether group, or a divalent organic group having 2 to 50 carbon atoms; when n01 is an integer other than 2, w represents an n01-valent organic group having 2 to 50 carbon atoms; and y represents a single bond or divalent connecting group having 1 to 10 carbon atoms and optionally having an oxygen atom.