The temporary anion states of gas-phase furan, isoxazole, oxazole, pyrrole, pyrazole, imidazole, thiophene, isothiazole, and thiazole are characterized by means of electron transmission spectroscopy. The measured energies of vertical electron attachment are compared with the virtual orbital energies of the neutral state molecules supplied by MP2 and B3LYP calculations with the 6-31G* basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the experimental attachment energies. Replacement of a ring CH group with a nitrogen atom increases the electron-acceptor properties, although the stabilization of the pi* anion states is not as large as that of the pi cation states, in line with the bond length variations caused by aza-substitution. In the spectra of thiophene and isothiazole the first pi* resonances display sharp vibrational structure with energy spacing of about 80 meV. The spectrum of isothiazole presents clear evidence for a low-energy (1.61 eV) resonance ascribed to the lowest sigma* anion state.