Coincidence measurements between photoelectrons and photoions produced by 21.21 eV quanta from helium are described. The photoelectrons are energy analysed by a retarding field spectrometer, the photc'ons mass analysed by a magnetic mass spectrometer. The coincidence curves show the fragmentation of a molecular ion as a function of the internal energy of that ion. Molecules investigated are H2O, NH3, CH4, CD4, C2H2, C2H4, C2H6, HCOOH, and CH3OH. In some cases fragmentation does not seem to occur completely, though the internal energy of the ion is sufficient