The threshold voltage at which visible defects could be produced in MgO by electron irradiation in a HVEM has been determined over a wide range of temperature for the ⟨001⟩, ⟨011⟩ and ⟨111⟩ directions. In a separate experiment, the growth of oxygen vacancy centres produced by electron irradiation along ⟨001⟩ at 90 K was followed by optical absorption measurements for several energies between 325 kV and 500 kV. The oxygen vacancy growth measurements gave an upper limit to the threshold voltage of 300 kV which is equivalent to a value of the threshold displacement energy Eα for oxygen of 53 eV. The HVEM measurements over the temperature range 125-1070 K gave an effective threshold voltage which varied considerably with temperature. The effective threshold at room temperature and below occurred at 460 kV which is considerably higher than the optically determined oxygen threshold and must therefore correspond to the magnesium threshold. This gives a value of Edfor magnesium of 60 ± 3 eV for all three directions. At higher temperatures the effective threshold voltage fell to give minima at ∼700 K for each of the three orientations. Comparison with the optical measurements suggests that the minima are the thresholds for oxygen displacement. It is proposed that the decrease in the effective threshold is a defect mobility effect which allows dislocation loops to be formed from the displaced oxygen ions only. The minima at 700 K would therefore correspond to values of Eα for oxygen of 44±3 eV for ⟨001⟩, 64 ± 3 eV for ⟨011⟩ and 46 ± 3 eV for ⟨111⟩. At higher temperatures for ⟨001⟩ and ⟨011⟩ the effective thresholds rose to a peak at ∼900 K of 340 kV and 410 kV respectively.