Reactive diffusion between iron and silicon wafer has been studied by using 99.995% electrolytic iron flakes (4.5N–Fe), a 99.5% iron sheet (3N–Fe) and two kinds of iron plates having different oxygen contents which were made by melting the same 99.9% electrolytic iron flakes under different conditions. The growth rate of the Fe₃Si diffusion layers formed in the 4.5N–Fe/Si diffusion couples is faster than that in other couples. The authors have previously reported briefly that Fe₃Si formed in bulk Fe/Si diffusion couples consisted of interstitial free iron (IF-steel) and Si wafer grows considerably faster than that in bulk 99.99%Fe (4N–Fe)/Si diffusion couples and proposed a possible explanation that oxygen atoms which initially distribute randomly in the 4N–Fe specimens are enriched in the iron near the Fe/Fe₃Si interface and they react with Si atoms and form SiO₂ film on the surface of voids formed in the iron near Fe/Fe₃Si interface. The SiO₂ film acts as a diffusion barrier and slows down the growth rate of iron silicides. Ti and Al atoms in IF-steel remove the effect of oxygen by scavenging oxygen atoms around them. So, Fe₃Si in the IF-steel/Si couple can grow faster than that in the 4N–Fe/Si couples. This explanation suggests that the growth rate of Fe₃Si is very fast in a Fe/Si diffusion couple, oxygen concentration in the iron is extremely small. The present experimental result that the growth rate of Fe₃Si is faster than the IF-steel/Si diffusion couples supports this explanation because the oxygen concentration in 4.5N–Fe is very small.