Two isostructural series of polycrystalline compounds: Er_2-xY_xFe₁₄B and Er_2-xCe_xFe₁₄B have been studied by ⁵⁷Fe Mössbauer spectroscopy in the temperature range 80-370 K. The spin reorientation phenomenon (a transition from basal plane to axial easy magnetisation direction) has been studied extensively by a narrow step temperature scanning in the vicinity of the transition. Using the procedure of subtracting the Mössbauer spectra taken for the same compound at different temperatures, it was possible to follow the influence of transition on the shape of spectra. From this procedure it was concluded that in the region of transition each subspectrum splits into two Zeeman sextets, which are characterised by different hyperfine magnetic fields and quadrupole splittings. The consistent way of describing the Mössbauer spectra was proposed. The spin reorientation temperatures have been established for all compositions and compared with the values obtained from theoretical calculations of spin orientation angle based on phenomenological model. The spin arrangement diagrams have been constructed.