Experiments on the formation of negative ions by electron bombardment of hydrogen, oxygen, and water vapor at pressures of 0–4 mm are described. The only ion observed in hydrogen is H−, formed by dissociative attachment. Pressure dependence of the O2−intensity shows that a secondary collision is involved in its formation. Data on the formation of O−and O2−at low electron energies indicate that the secondary collision leading to the formation of O2−does not involve an O−ion. The formation of O2−probably involves a stabilizing collision with a molecule of gas after the initial formation of the ion by electron attachment. From the behavior of H−, O−, and OH−intensities with H2O pressure, it is shown that the secondary process for the formation of OH−in water vapor is probablyH−+H2O→OH−+H2.The cross section for this reaction is more than 10−17cm2.