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-d₃ (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.