A detailed mechanism of the molecular split of ethylene into acetylene and hydrogen, which occurs when ethylene is photosensitized by Hg(3P1), is presented. It is suggested that the triplet ethylene molecule which is first formed isomerizes to a molecule of uncertain lifetime in which two hydrogen atoms act as "bridges" between the two carbon atoms. The hydrogen molecule which splits from the ethylene molecule is formed when these two atoms leave simultaneously. The experimental observations on the decomposition of ethylene when photosensitized by Cd(1P1) or Zn(1P1) are consistent with the primary interaction of the excited metal atom with either a C–H group or with the π electrons.