The UV absorption spectra of the charge transfer (CT) complexes formed by iodine with C2H2NH, C2H2O and C2H2S were studied in order to interprete the first ionization potential of these organic molecules by using the empirical relationships hn(CT/Max)=f(ID). Taking into account the basical specificity of these relations, the trend of the CT data shows C2H2NH and C2H2O to be n-donors towards iodine and the first ionization potential of the same donors could be ascribed to the removal of an electron from a non-bonding orbital localized on the heteroatom. Theoretical calculations however do not support this interpretation in the case of C2H2NH