Although the literature contains many measurements of the diffusivity of carbon in Fe and steels, recent progress in plasma carburizing has made this regime around 1050 °C and over 1% carbon concentration of great interest. We have measured the diffusivity of carbon in steel from 975 to 1075 °C up to 1.3% carbon concentration by the steady‐state method. Our results are in agreement with previous measurements of Smith and Wells, Batz, and Mehl in the regime of low temperatures and carbon concentrations. Using activation energies determined over a wider temperature range by the latter authors, we derive as an empirical expression forDthroughout the range of temperatures and concentrations investigated,D=0.47 exp (−1.6 C) ×exp[−(37 000−6600 C)/RT] cm2/sec. Here,Cis expressed in weight percent carbon and the energies are in cal/mole.