The microstructure of binary Co-13.6 wt% Cu and Cu-4.9 wt% Co alloys after long anneals (930-2,100 h) was studied between 880 and 1,085 A degrees C. The contact angles between (Co) particles and (Cu)/(Cu) grain boundaries (GBs) in the Cu-4.9 wt% Co alloy are between 50A degrees and 70A degrees. In the Co-13.6 wt% Cu alloy, the transition from incomplete to complete wetting (coverage) of (Co)/(Co) GBs by the second solid phase (Cu) has been observed. The portion of completely wetted (Co)/(Co) GBs increases with increasing temperature beginning from T (wss) = 970 +/- A 10 A degrees C and reaches a maximum of 15% at 1,040 A degrees C. This temperature is very close to the Curie point in the Co-Cu alloys (1,050 A degrees C). Above 1,040 A degrees C, the amount of completely wetted (Co)/(Co) GBs decreases with increasing temperature and reaches zero at T (wsf) = 1,075 +/- A 5 A degrees C. Such reversible transition from incomplete to complete wetting (coverage) of a GB by a second solid phase is observed for the first time.