The trapping of positrons at vacancy site in some materials provide a new and sensitive method for the equilibrium determination of point defect migration enthalpy. Data are presented for commercial Al-Mg alloys and fitted to a model allowing presentation in the form of Arrhenius plots, hence the migration enthalpy H m iv can bedetermined by the positron annihilation lifetime technique. The results show that the value of Hm iv increases as the concentration of Mg increases.