Anions of mixed cluster between carbon dioxide (CO2) and pyridine (C5H5N, denoted by Py), each possessing a negative adiabatic electron affinity, were studied. The minimum size cluster anion was the 1:1 complex of [(CO2)(1)(Py)(1)](-), which displayed highest mass spectral intensity in the series. It was shown by photoelectron spectroscopy that [(CO2)(1)(Py)(1)](-) is not a typically expected ion-molecule complex of the type (CO2)(1)(-)(Py)(1) or (CO2)(1)(Py)(1)(-). Ab initio calculation in the UHF/6-311++G** level revealed that [(CO2)(1)(Py)(1)](-) is indeed a totally new anion of the type (C5H5N-CO2)(-) with a planar C-2v structure, bound by an intermolecular bond of a strongly covalent character. The calculation predicted 1.24 eV for the vertical detachment energy of this anion, which is in reasonably good agreement with the measured value of 1.46 eV by photoelectron spectroscopy. A Mulliken analysis showed that the excess negative charge is fully delocalized throughout the entire molecular frame. Further calculation on intramolecular rotational barrier predicted a considerable barrier height of similar to 0.68 eV, which could be due to an additional, weak pi-bond character of the newly formed C-N sigma-bond. Such extra pi-bond network was suggested to provide a path for the charge delocalization and thus lead to the stability of the anion core.