Abstract: A method of treating a subterranean formation, for example a subterranean formation susceptible to formation of a undesirable gel-like goo, is described. The method comprises contacting the formation with a fluid (A) comprising a polymer (PP) which includes a pendent group which includes a moiety O—S?O??(I) wherein the sulphur atom of the moiety is covalently bonded, optionally via a linking atom or group, to the backbone of the polymer.