By mass spectrometric study of the appearance of C − 2, CH − and C 2H − from CH 4, C 2H 2, C 2H 4, C 2H 3X (X = Cl, F) the following electroaffinities have been determined: EA(CH) = (2.6 ± 0.3) Ev, EA(C 2) = (3.3 ± 0.2) eV and EA(C 2H) = (2.1 ± 0.3) eV. Some evidence is brought to the exiatence of electronic excited states of these ions. An evaluation of these negative ions dissociation energy in their ground electronic state is given and is found to be of the order of magnitude of the corresponding isoelectronic molecular species dissociation energy.