Using low energy electron microscopy, diffraction and x-ray photoemission electron microscopy, we study the phase separation in an Fe-Ni alloy film on W(110) at around 30 at.%Ni and at monolayer thickness. At high temperature, the monolayer-thick alloy is shown to transform into a biphase with submicron regions of different surface density exhibiting (1×1) and (1×8) structures. The former is pseudomorphic to the bcc substrate, whereas the latter is a slightly-distorted hexagonal adlayer lattice reminiscent of an fcc(111) monolayer. The stoichiometries of the two monolayer phases are Fe0.85Ni0.15 and Fe0.58Ni0.42 in laterally resolved x-ray photoemission microscopy measurements, with the bcc phase rich in Fe compared to the fcc one. This heterogeneous surface can be viewed as the two-dimensional limit of the bcc-fcc phase separation observed in thick films and in bulk Fe–Ni near the same composition. The length scale associated with the lateral heterogeneity in the monolayer film is much larger than the one observed in bulk alloys, suggesting that surface transport is the key mechanism in the kinetics of the phase separation process. [DOI: 10.1380/ejssnt.2015.256]