We have studied the effect of plastic deformation by compression on the occurrence of discontinuous precipitation in Al-30% Zn alloy after ageing at two different temperatures (348 and 423 K) has been studied. Optical microscope, scanning electron microscope, X-ray diffraction and differential scanning calorimetry were used for characterization. During ageing of undeformed alloy, the grain boundary of supersaturated solid solution represents the favorite site of precipitation by discontinuous mechanism. We found that the occurrence of discontinuous precipitation depends mainly on the degree of plastic deformation before ageing. The grain boundary act as reaction front and migrates in case of low degree of prior deformation. (c) 2005 Springer Science + Business Media, Inc.