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 Mg-32(Al,Zn)(49) (tau phase), MgZn2 (eta phase), AlMg4Zn11 (eta' phase), and Mg7Zn3 phases embedded in the matrix of Al-based solid solution. During heating in differential scanning calorimeter (DSC), all these phases dissolved around 148 A degrees C. The tau nanoparticles coherent with (Al) matrix-formed instead around 222 A degrees C. HPT of the as-cast alloys strongly refined the grains of (Al) solid solution from 500 mu m to 120-150 nm. The particles of tau, eta, eta', and Mg7Zn3 phases fully dissolved in the (Al) matrix. During the following DSC-heating, particles of eta phase appeared and grew. Their amount became maximal around 166 A degrees C. The growth of eta phase in the fine-grained HPT-treated alloys instead of tau phase in the coarse-grained ones is explained by the shift of the (Al) + eta/(Al) + eta + tau/(Al) + tau lines in the Al-Zn-Mg ternary phase diagram due to the grain boundary (GB) adsorption. At 166 A degrees C the eta phase formed the continuous flat layers in numerous (Al)/(Al) GBs. This corresponds to the complete GB wetting by the eta phase. Other (Al)/(Al) GBs contain separated lenticular eta particles (incomplete GB wetting). Increasing the temperature from 166 to 320 A degrees 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 eta phase proceeds between 166 A degrees C and 320 A degrees C.