Phase equilibria in the Ni-Al-Si system have been studied in the temperature range 550°C using a ternary diffusion couple experiment with pure Ni and an Al-Si eutectic alloy. A number of binary and ternary intermetallic compounds have been observed. The intermetallics NiAl$_3$, Ni$_2$Al$_3$, NiAl and Ni$_3$Al form as solid layers just above pure Ni. Each of the binary intermetallics has some solubility for Si, which replaced Al in the ordered lattices. Two ternary intermetallics τ$_1$ and τ$_3$ have been observed. The τ$_1$ phase has a composition very close to Ni$_4$AlSi and τ$_3$ (Ni$_3$(Al$_{1-x}$Si$_x$)$_7$) has been analyzed to have a lower Si content. A new ternary phase Al(Ni)Si$_4$ (τ$_4$) has been observed in the diffusion couple heat treated for longer times at 550°C. The morphology of the binary and ternary phases is presented.