Electron attachment to 1,4-chlorobromobenzene yields the ionic fragments Cl- and Br- appearing in the energy range approximate to0-1 eV with Br by about a factor of 70 more intense at room temperature. Increasing the temperature of the target molecule up to 540 K results in an unexpected temperature dependence of the cross-section in that it first increases and after passing a maximum (400 K for Br- and 460 K for Cl-) decreases. This behaviour is qualitatively interpreted by the temperature dependent population of the relevant normal modes containing the C-Cl and C-Br stretch vibration.