The temperature dependence of the rate coefficients for thermal energy dissociative electron attachment reactions has been measured for nine halomethane compounds. Rate coefficients for CCl4, CFCl3, CF2Cl2, CF3Cl, CHCl3. CH2Cl2, CHFCl2, CH3I, and CH2Br2 were determined between 300 and 800 K from relative rate measurements by using SF6 as a reference compound. A flowing afterglow-electron cyclotron resonance technique was employed. Mass spectral analysis of product negative ions confirmed a dissociative attachment mechanism. Semiempirical quantum mechanical calculations of the geometry change associated with negative ion formation were carried out.