Metastable precipitates play important roles in a super high strength Al-Zn-Mg based alloy Mesoalite ®. The present study was undertaken to examine the composition of the metastable precipitates in Mesoalite ® and the effect of Mn addition on the composition. The Zn+Mg concentration of hЈ metastable precipitates detected by 3DAP three dimensional atom probe was ϳ25% in the alloy Al-4.1Zn-3.1Mg-0.57Cu-0.01Ag at%. By comparing with the results of G.P. zones in an Al-Zn alloy, it was suggested that the intrusion of Al atoms from the matrix lowered the Zn+Mg concentration of metastable precipitates detected by 3DAP. A model to quantify the intrusion of Al atoms was developed and applied to the metastable precipitates. The Zn+Mg concentration was quantified to be ϳ65 at% at 383 K and ϳ75 at% at 413 K. When the alloy contained Mn, furthermore, it was suggested that Mn affected the Zn/Mg ratio of metastable precipitates by lowering the virtual Zn/Mg ratio of the specimen composition.