[en] Using the flowing afterglow Langmuir probe (FALP) technique, dissociative attachment coefficients β for reactions of electrons with gas phase superacids HCo(PF₃)₄, HRh(PF₃)₄ and carbonyl hydride complexes HMn(CO)₅, HRe(CO)₅ have been determined under thermal conditions over the approximate temperature range 300∼550 K. The superacids react relatively slowly (<1/20 of β_max) with free electrons in a thermal plasma, and the values of β obtained this far do not show a correlation between acidity and β. The pioneer researchers in this field had speculated that any superacid would be a rapid attacher of electrons; it was found that this speculation is not true in general. The product distribution of electron attachment reaction to HCo(PF₃)₄ was found to be independent of temperature even though the β[HCo(PF₃)₄] increases with temperature. This proposes that the electron attachment process occurs well before the excited complex dissociates. In addition, the activation energy of HCo(PF₃)₄ for electron attachment has been derived from the Arrhenius plots. The carbonyl hydride complexes, HMn(CO)₅ and HRe(CO)₅, react relatively rapidly (>1/4 of β_max) with free electrons in thermal plasma. This indicates that these reactions cannot be significantly endothermic. Observation of rapid attachment for these non-superacids shows that the Mn-CO and Re-CO bonds are weaker than the Mn-H and Re-H bonds, respectively. Comparisons between the carbonyl and trifluorophosphine cases implies that fast electron capture is related more to the CO ligand than to the transition-metal species