The phase transformation at 480°C of a Fe-0.17 Ni-0.027 W (at.%) alloy was studied by anomalous small-angle X-ray scattering (ASAXS). Partial structure factors (PSF) were extracted from ASAXS data obtained in the neighbourhood of K Fe and L IIIW absorption edges. The PSF were decomposed into two contributions: a pseudobinary one and a complementary one. This latter was shown to be very low ( ∼- 3%). From these PSF, it has then been concluded that the precipitation occurs according to a two-phase system. Such a result agrees well with TEM observations which have revealed two phases having different crystalline structures: precipitates of hexagonal symmetry embedded in a b.c.c. depleted ex-martensitic matrix. The composition of both phases does not change during isothermal ageing, being for the precipitate C Fe = 0.956, C W = 0.044, C Ni ∼-0 and for the matrix C Fe = 0.696, C Ni = 0.291, C w = 0.013 with a volume fraction of precipitates of about 45%.