Provided herein is a new form of 3-fluoro-7-(2,2,2-trifluoroethoxy)phenoxathin 10,10-dioxide, which demonstrates higher stability relative to other forms of 3-fluoro-7-(2,2,2-trifluoroethoxy)phenoxathiin-10,10-dioxide. In particular, this new form affords less dosage critical administration of 3-fluoro-7-(2,2,2-trifluoroethoxy)phenoxathin 10,10-dioxide relative to other forms. The new solid form of 3-fluoro-7-(2,2,2-trifluoroethoxy)phenoxathiin-10,10-dioxide has been determined and is provided herein. This new solid form of 3-fluoro-7-(2,2,2-trifluoroethoxy)phenoxathiin-10,10-dioxide can be characterized by any of a number of its properties, including, but not limited to, melting point, differential scanning calorimetry, infrared spectroscopic spectrum or portions thereof, solubility, methods and conditions under which this form is prepared, and/or precipitated from solution, and, when in crystalline form, the crystalline form can be characterized according to the diffraction pattern or portions thereof