This study is aimed to point out the important role played by resonance electron attachment in reductive dehalogenation, in particular in phytoremediation of organic pollutants under conditions of excess negative charge. To model enzymatic reactions occurring in reductive conditions, low-energy electron capture by the halogenated herbicides atrazine and bromoxynil was studied in vacuo using electron transmission spectroscopy. A variety of decay channels of the temporary molecular negative ions was discovered by means of dissociative electron attachment spectroscopy. The experimental results were interpreted with the support of quantum-chemical calculations. Dehalogenation of atrazine and bromoxynil was found to be the dominant decay of the molecular negative ions formed at thermal energies of the incident electrons. It is concluded that formation of negative ions by electron donation in enzymatic active centers followed by their dissociation along the σ bond can be considered as the main mechanism of reductive dehalogenation.