According to the invention, a fully viscosified acid diversion system for hydrofluoric acid has been developed. The viscosifying agent comprises a xanthan polymer or a derivative thereof. A method of acidizing a portion of a subterranean formation is provided, the method comprising the steps of: (A) forming a viscosified treatment fluid comprising: (i) water; (ii) hydrogen fluoride or a controlled-release source of hydrogen fluoride; and (iii) a gelling agent comprising a xanthan polymer or derivative thereof; and (B) introducing the viscosified treatment fluid into the portion of the subterranean formation. A breaking agent can be used to achieve a controlled gel break time under downhole conditions. Other additives can also be included in the treatment fluid. A composition for use in treating a subterranean formation is also provided, the composition comprising: (A) water; (B) hydrogen fluoride or a controlled-release source of hydrogen fluoride; and (C) a gelling agent that comprises a xanthan or derivative thereof.