The microstructures of precipitates in Al–Zn–Mg alloys in peak-aged condition have been studied using scanning transmission electron microscope. The same thermo-mechanical treatment was applied in all alloys. Investigation of peak-aged samples revealed that the most commonly found phases were η′ and η₁ with their respective habit planes on {111}_Al and {100}_Al. η′ phases under [110]_Al were analyzed and compared with η′ structure models. Furthermore, a close inspection of η₁ phase as the second most found precipitate revealed that it incorporates an anti-phase resembling boundary, not observed in other orientation relationships that precipitates create with Al matrix, in addition, differences in matrix-precipitate interfaces between η′/η₂ and η₁ phases were noticed. This paper addresses the first part to the analysis of η′ phase. Next part is extended to the analysis of the η₁ phase.