Plastic zone sizes (PZS, rp) of high purity peak-aged Al-Zn-Mg alloys with different dispersoid phases have been measured by using selected area channelling patterns. Compact tension specimens were loaded to predetermined stress intensity values Keff and the plane strain plastic zones were mapped. When the plane strain PZS are plotted against the function (Keffσys)2, all the data fall in a narrow band. This suggests that the crack sharpness is independent of slip homogenization in the matrix and the crack initiation is intergranular which is confirmed by the micrographs of mid-sections of the samples. The slope of the rp versus (Keff/σys)2 line is found to be 0.033 which is in close agreement with the theoretically calculated value.