The UV absorption spectrum of ethylene during the pulse heating of an ethylene–argon mixture to prepyrolysis temperatures in the range 950 < Т < 1285 K was studied by kinetic spectroscopy in a free-piston adiabatic compression unit. New intense light absorption bands were found at 210 < λ < 260 and 440 < λ < 490 nm. Ab initio quantum-chemical calculations of the mechanism of the thermal cis-trans isomerization of ethylene were performed. When this mixture is heated, the point symmetry group of the ethylene molecule in the ground state S0 reduces to С1, which is characteristic for ethylene at the minimum of the S1 state, due to the second order Jahn–Teller effect.