The structure, chemistry and magnetic domain configurations of YFe11Mo polycrystalline aggregates have been investigated by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and Lorentz microscopy. Although extensive reports on the magnetic properties of this compound can be found in the literature, its exact equilibrium structure remains elusive. The compound was prepared by arc-melting followed by splat-quenching and subsequent annealing. The quenched material consisted of a ThMn12-type matrix and alpha-Fe(Mo) impurity phase. Annealing lead to iron segregation from the matrix, as evidenced by a clear shift to higher angles of the alpha-Fe(Mo) peak observed in the diffraction pattern. Nevertheless, the composition variation in the intermetallic compound was accompanied by an extremely limited evolution of the lattice parameters, suggesting the presence of Fe vacancies after annealing. Rietveld analysis carried out on the annealed material points to Fe vacancies at the 8i sites. The present results indicate that the fraction of point defects in the ThMn12-type structure adapts to the processing route and that the stable configuration depends on the temperature. YFe11Mo grains present internal domain walls forming stripe/maze patterns characteristic of high anisotropy materials, while residual alpha-Fe(Mo) grains exhibit vortex configurations. Annealing hardly affected the magnetic domain configurations of the YFe11Mo and alpha-Fe(Mo) aggregates.