Abstract: The variation of carbon composition in undercooled austenite during bainitic incubation period has been investigated in 40CrMnSiMoVA by scanning auger microprobe (SAM). The results show that the outstanding diffusion and redistribution of carbon occur in isothermally reacted specimens. This leads to the formation of carbon-poor regions (CPR) near austenite grain boundaries and in austenite grains. The carbon content in CPR exceeds the equilibrium carbon composition of ferrite. Furthermore, it is indicated that the existence of CPR is thermodynamically possible.