The temperature dependence of the positron annihilation peak coincidence rate has been measured in iron from 20 degrees C to 1320 degrees C. The peak rate did not change across the ferro-para magnetic transition at approximately 770 degrees C but decreases by 1.5% across the alpha - gamma phase transition at 910 degrees C. The trapping model analysis of the data yields values of the vacancy formation energies (E_f) of 1.4+or-0.1 eV in alpha -Fe and 1.7+or-0.2 eV in gamma -Fe. The correlation between E_f and T_t, where T_t is the threshold temperature, has been derived from the trapping model.