An investigation of the wettability of the intermetallic compound NiAl by a liquid Ni-4.5 wt% Si-3.2 wt% B filler metal is presented in this paper. Dynamic observations of spreading of Ni-Si-B droplets, conducted using hot-stage light microscopy, are correlated with post-cooling microscopy and analysis. The paper examines the influence of the oxide layer on the NiAl substrates, on the progression of spreading of the Ni-Si-B liquid. Termination of spreading of the Ni-Si-B droplets by the onset of isothermal solidification at the spreading front is considered. Spreading of the Ni-Si-B droplets was found to be rapid until the onset of isothermal solidification at the spreading front. However, once isothermal solidification commenced, negligible further spreading was observed. The Ni-Si-B filler metal was observed to spread by undermining of the substrate oxide. However, a marked reaction occurred between the substrate oxide and the Ni-Si-B filler metal. This reaction served to remove the substrate oxide layer. The paper contrasts the mechanisms of substrate oxide undermining and isothermal solidification of liquid Ni-Si-B droplets on NiAl with those occurring during the spreading of the same liquid on pure nickel and Ni-Cr alloys.