The diffusivity of Ni in Fe-Ni and Fe-Ni-P martensite,\(D_{\alpha _2 }^{N_1 } \), has been determined between 700 and 300 °C using electron microprobe (EMP) and scanning transmission electron microscope (STEM) techniques. Alloys of various bulk compositions (0 to 30 wt pct Ni, Fe) were homogenized in the single phase austenite (γ-fee) field and quenched to form martensite, α2 (bcc). Appropriate alloys were tempered isothermally at 300 to 700 °C. The γ nucleated and grew in the parent α2. The composition of the γ phase and the concentration gradients in the α2 were measured with the EMP andJor STEM. In order to determine\(D_{\alpha _2 }^{N_1 } \) experimentally measured Ni concentration gradients were matched to Ni concentration gradients calculated by a simulation model. The calculated gradients were obtained by solving the appropriate form of Fick’s second law using the Crank-Nicholson numerical technique. The observed diffusivities varied with temperature. Above approximately 410 °C,\(D_{\alpha _2 }^{N_1 } \) while below 410°C,\(D_{\alpha _2 }^{N_1 } \) = (2.27 × 10−15) exp (− 10,600/RT) cm2/s. The effect of P is to increase the Fe-Ni diffusivities at any temperature by the factor (1 + 1.27Cp + 0.623Cp2) whereCp is the amount of P (wt pct) in α2. The discontinuous diffusion behavior of\(D_{\alpha _2 }^{N_1 } \) is attributable to the high dislocation density of the α2. Above approximately 410 °C lattice diffusion is dominant while below 410 °C dislocation pipe diffusion is dominant.