The microstructural changes of a commercial 90W-5Ni-5Fe heavy alloy as a function of thermal treatment have been examined using a variety of metallographic techniques. Upon solution treating at 1350 °C for 1 hour, quenching, and subsequently aging at 750 to 850 °C for various times, a discontinuous precipitation reaction was observed in the matrix phase. Concurrently, precipitation of a face-centered cubic phase with lattice parametera ₀ = 1.096 ± 0.004 nm occurred at the interphase boundaries between tungsten grains and the matrix. Metallographic examination of two specially manufactured “matrix-composition” alloys, which have been heat treated in a similar manner, provided additional information concerning these respective precipitation reactions. The products of the discontinuous reaction were found to be tungsten and solute-depleted matrix phase in both the heavy alloy and a matrix-composition alloy. Moreover, an M₆C eta-carbide (M = Ni, Fe, and W) obtained in a second matrix-composition alloy was found to have the same crystal structure as the interfacial phase in the heavy alloy which is believed to be an M₁₂C eta-carbide in agreement with previous results.