The method described in the preceding paper [Gerold (1977),J. Appl. Cryst.10, 25–27] has been applied to the system Al–Zn–Mg where the small‐angle scattering has been measured from nine alloys having zinc and magnesium contents up to 5.4 and 2.5 at. %, respectively. The alloys were annealed at room temperature where the metastable equilibrium of GP zone formation is reached after about one month. From combined analysis of the X‐ray and neutron results the slopes of the tie‐lines have been evaluated in this region of the phase diagram. The further analysis leads to the approximate determination of the miscibility gap limits. The confidence limits of the determination were restricted by it not being possible to make measurements far enough out in the tail of the neutron scattering profile from the small GP zones (diameter less than 30 Å) to allow the background intensity to be separated completely satisfactorily. The ultimate limiting factor for this type of analysis may be the accuracy to which the mean atomic v