A new phase transition in an intermetallic compound K2Cs has been found at about 110K. Both X-ray scattering and resistivity measurements show an enormous change in volume ( Delta V/V approximately 0.14) at the transition temperature. The samples of K2Cs appear to be extremely soft and they recover completely after the strong deformation. In the present letter the authors suggest that these effects are connected with the electronic transition in Cs atoms discussed by R.L. Sternheimer (1950).