The crystal structure of the new polymorphic modification of B(C6F5)(3)Py complex was determined. It was shown by a static tensimetric method using a membrane null manometer that in the condensed phase in an excess of pyridine, B(C6F5)(3)Py complex is stable up to 220 degrees C. When temperature rises to 230 degrees C, the complex undergoes irreversible thermal decomposition, with the release of pentafluorobenzene and the polymerization of pyridine. The sublimation enthalpy of B(C6F5)(3)Py was determined for the first time by the calorimetric method. The results of quantum chemical calculations using the M06-2X method provide qualitative agreement with the experiment, in contrast to the B3LYP method.