X-ray measurements cannot give a complete microscopic interpretation of the structure of carbon—iron martensite because of the broad diffuse diffraction lines. They show that the iron martensite has a body-centered tetragonal lattice on the average, but cannot give an exact picture of the distortion centers around the carbon atoms. Several samples of martensite were studied by means of the Mössbauer effect to separate the octahedra occupied by carbon atoms from the rest of the lattice. The iron atoms around each octahedron gave two extra different hyperfine fields, with an isomer shift relative to Armco iron. The strength of the new fields coupled with the intensity ratio of the absorption lines leads to the interpretation that the distortion around a carbon atom is local, and that most of the crystal has an almost bcc structure, probably with a small strain field centered at the carbon atom.