The aim of this work is thestudy of grain boundary wetting phase transitions in the binary W –Ni alloys. Influence of the alloys composition and annealing temperature on the structure of grain boundaries has been investigated. The study was carried out by means of high-resolution electron microscopy (HREM) and analytical electron microscopy. Different kinds of wetted GBs were revealed by HREM, namely (i) “dry” GBs (incomplete wetting), (ii) GBs with thick (complete wetting) and (iii) thin layers of bulk Ni-rich phase (pseudo-incomplete wetting). The continuation of tie-line of the GB wetting phase transition into one-phase (W) area is constructed in the bulk W–Ni phase diagram. Authors thank the Russian Foundation for Basic Research (contract 18-03-00284) for the financial support.