The iron-57 Mössbauer spectra of YCoFe3B have been measured between 4.2 and 480K and reveal that YCoFe3B exhibits an axial orientation of the iron magnetic moments below 450K and a basal orientation above 450K. This spin reorientation, also observed in the thermomagnetic curves, results from the different signs of the contributions to the magnetic anisotropy of the 2c and 6i sites that are occupied by iron. The neutron diffraction patterns of YCoFe3B have been measured at 2K and between 290 and 770K and have been successfully analyzed with a model compatible with the magnetic orientation obtained from the Mössbauer spectra. The hybridization between the cobalt or iron 3d orbitals and the boron 2p orbitals leads to a larger magnetic moment and hyperfine field on the 2c site as compared to the 6i site. © 2009 IOP Publishing Ltd.