The structure, phase composition, and their thermal evolution were studied in case of ternary Al-Zn-Mg alloys before and after high-pressure torsion (HPT) in Bridgman anvils. The as-cast non-deformed alloys contained the fine particles of Mg₃₂(Al,Zn)₄₉ (τ phase), MgZn₂ (η phase), AlMg₄Zn₁₁ (η′ phase), and Mg ₇Zn₃ phases embedded in the matrix of Al-based solid solution. During heating in differential scanning calorimeter (DSC), all these phases dissolved around 148 C. The τ nanoparticles coherent with (Al) matrix-formed instead around 222 C. HPT of the as-cast alloys strongly refined the grains of (Al) solid solution from 500 μm to 120-150 nm. The particles of τ, η, η′, and Mg₇Zn₃ phases fully dissolved in the (Al) matrix. During the following DSC-heating, particles of η phase appeared and grew. Their amount became maximal around 166 C. The growth of η phase in the fine-grained HPT-treated alloys instead of τ phase in the coarse-grained ones is explained by the shift of the (Al) + η/(Al) + η + τ/(Al) + τ lines in the Al-Zn-Mg ternary phase diagram due to the grain boundary (GB) adsorption. At 166 C the η phase formed the continuous flat layers in numerous (Al)/(Al) GBs. This corresponds to the complete GB wetting by the η phase. Other (Al)/(Al) GBs contain separated lenticular η particles (incomplete GB wetting). Increasing the temperature from 166 to 320 C led to the disappearance of the completely wetted (Al)/(Al) GBs. In other words, the transition from complete to the incomplete wetting of (Al)/(Al) GBs by the η phase proceeds between 166 C and 320 C.