Carbon and nitrogen austenites, modeled by Fe8N and Fe8C superstructures are studied by full-potential LAPW method. Structure parameters, electronic and magnetic properties as well as hyperfine interaction parameters are obtained. The results prove that chemical Fe-C bond in Fe8C has higher covalent part than in Fe8N. The calculation of electric field gradient allowed us to carry out a good interpretation of Mossbauer spectra for Fe-C and Fe-N systems.