Precipitation of pure polymorphic forms (I and II) of salmeterol xinafoate (SX) in supercritical fluids was investigated as a function of operating pressure and temperature. It has been shown that the formation of each polymorph is governed by both thermodynamic shift and kinetic effects, which are closely associated with the extent of miscibility between the supercritical CO(2) and methanol cosolvent. In addition, the surface energetics of SX exhibit a sharp discontinuity at the transition point in concordance with the particular polymorphic form generated, being essentially independent of the temperature or pressure below and above this point. The conditions of complete miscibility of the two solvent phases involved are critical for the formation of SX Form II.