The detailed investigations of the crystal structure of indium-tin intermetallic compounds revealed at low temperature a phase transition of In Sn. It is suggested that this phase transition is responsible for the "quench effqct", i. e. the difference of transition temperatures to the superconducting state, depending on the quenching 3 on the annealing of specimens, a s described by Merriam and Von Herzen (1).Single crystals of In Sn composition were grown from the melt of indium and tin 3 99.99% pure. It was known that the crystal structure of In Sn consists of a simple body centred tetragonal lattice cell (2) o r face centred tetragonal cell in another unconventional setting (3). The X-ray measurements performed in our laboratory on an automatic single crystal diffractometer with graphite monochromated molybdenum radiation confirmed these results in the temperature range from room temperature down to -25 C. If the cooling goes on sufficiently quickly, the room temperature crystal structure is preserved even at -80 C, but the crystal cooled slowly down to -40 C undergoes the phase transition resulting in a quite different and more complicated crystal structure. Table 1. The transition is reversible except that the specimen brought back to room temperatue is no more a single crystal, giving an X-ray pattern with spotty lines typical for powders with a considerable grain size, Single crystals of In Sn immersed in the liquid nitrogen and brought back to 3 room temperature remain, howeger , single crystals which proves that the high temperature phase can be easily overcooled.3 physica (b)