[en] We have studied here the distribution of carbon and alloying elements in the martensite-austenite (M-A) constituent in intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) of a high-strength pipeline steel using atom probe tomography (APT). Notable enrichment of C (0.49 wt%) and Mn (2.32 wt%) was observed within the M-A constituent, which induced the formation of lath martensite and deteriorated the toughness. Elemental segregation in the interfacial region between M-A constituent and matrix may contribute to the debonding mechanism of M-A constituent and assist nucleation of cleavage cracks. Distribution of solute Nb indicated no apparent difference between the matrix and M-A constituent.