The evolution of the Fe-Si phase’s formation during siliconising of ferrous material was studied. The examined coatings were formed at 990 oC, via chemical deposition of Si in fluidized bed reactor (FBR). The substrate used for this purpose was carbon steel, ST37. Coatings of 10-50 μm thickness were obtained. Optical micrograph (OM) SEM analysis, as well as XRD measurements were used for coatings characterization. During an annealing process of the as formed coatings an ordering of the solid solution of α-FeSi was observed. The ordering process was found to be continuous and the phase gradually becomes super saturated. A narrow portion of the X-ray diagrams including the 110 and the 220 reflections of the α -Fe and α-Fe3Si was chosen for data elaboration.