The kinetic energy distribution of ions produced by a dissociative ionization process is derived, taking into account the effect of thermal motion of the target molecule. In the case of dissociative attachment of monoenergetic electrons to a diatomic molecule the width at half maximum of the negative ion energy distribution is given by sq.rt.11BetakTE where Beta is the ratio of the mass of the ion to that of the parent molecule, T is the target gas temperature, and E is the most probable ion energy. Using a crossed field velocity filter O- ion energy distributions arising from the attachment of essentially monoenergetic electrons to O2 are studied as a function of electron energy at two gas temperatures. The measured widths of the distributions are consistent with the above relationship. Measurements of E as a function of the electron energy allow a determination of the electron affinity, A, of atomic oxygen. The result, A 1.5 plus or minus 0.1 eV, is in excellent agreement with photodetachment threshold determinations.