Interaction between ethylene and O– centres on magnesium oxide surfaces gives a radical having a nearly isotropic hyperfine coupling to two equivalent hydrogen atoms (58 G), a very weak coupling to a single proton, and strong coupling to 13C which indicates nearly unit spin-density in a 2p orbital on a single carbon atom. These results agree with expectation for H2CĊ– anions, isoelectronic with the relatively stable H2CN radical in every respect. We suggest that after initial abstraction to give H2CĊH radicals (not detected), ionization of the acidic acetylenic type proton occurs but this remains hydrogen-bonded to the radical anion: H2CĊ–—HO–. In contrast, reaction of ethylene oxide with F+s centres on the surface gave H2Ċ—CH2O– radicals.