Carbon-diffusion characteristics in metastable Г18C2 iron–manganese alloy after the cyclic $\gamma$–$\epsilon$–$\gamma$ (f.c.c.–h.c.p.–f.c.c.) martensitic transitions were investigated using the radioactive-isotope method. As shown, the process of carbon transport acceleration in the alloy phase-hardened by means of the $\gamma$–$\epsilon$–$\gamma$ transformations was caused by two independent mechanisms. They are as follow: athermal one realized via the stress field occurrence under the cyclic martensitic transitions and thermoactivated one realized during subsequent diffusion annealing due to low-angle subboundaries as well as the one-dimensional and two-dimensional crystal structure defects in f.c.c. austenite and h.c.p. $\epsilon$-martensite generated during these transitions. After the cyclic $\gamma$–$\epsilon$–$\gamma$ martensitic transformations, the diffusion coefficient of carbon at low temperatures (100–350°C) increased by more than three orders of magnitude. In this case, the diffusion coefficient at 325°C corresponds to the stationary diffusion coefficient at 900°C. Maximum carbon-diffusion coefficient rising was observed when number of thermal cycles was increased up to 100 and maximum structure defects density increasing was fixed. Phase composition variation was as an additional cause of carbon-diffusion characteristics changing during the $\gamma$–$\epsilon$–$\gamma$ cycling and subsequent diffusion annealing.