Ion-pair formation in gaseous nitromethane (CH3NO2) induced by electron transfer has been studied by investigating the products of collisions between fast potassium atoms and nitromethane molecules using a crossed molecular-beam technique. The negative ions formed in such collisions were analysed using time-of-flight mass spectroscopy. The six most dominant product anions are NO2-, O-, CH3NO2-, OH-, CH2NO2- and CNO-. By using nitromethane-d(3) (CD3NO2), we found that previous mass 17 amu assignment to O- delayed fragment, is in the present experiment may be unambiguously assigned to OH-. The formation of CH2NO2- may be explained in terms of dissociative electron attachment to highly vibrationally excited molecules.