The microstructure of binary Al-10 at% Zn and Al-15 at% Zn alloys after long anneals (800-4000 h) was studied between 190 and 258 A degrees C. The contact angles between (Zn) particles and (Al)/(Al) grain boundaries (GBs) were measured. They decrease with decreasing temperature. First (Al)/(Al) GBs completely wetted by the second solid phase (Zn) appear below T (wsAl0%) = 205 +/- A 5 A degrees C. Above T (wsAl0%) = 205 +/- A 5 A degrees C all (Al)/(Al) GBs are incompletely wetted by (Zn) solid phase. The extrapolation of the maximal contact angle theta to zero permits to obtain the T (wsAl100%) = 125 +/- A 10 A degrees C. Below this line all (Al)/(Al) GBs has to be completely wetted by (Zn) solid phase.