This thesis deals with (pre)precipitation phenomena occuring on tempering of iron-based interstitial martensites: iron-carbon (FeC), iron-nitrogen (FeN), iron-nickel-nitrogen (FeNiN), and iron-carbon-nitrogen martensites (FeCN). Martensitic specimens were prepared from pure iron or pure iron-nickel alloy by gaseous -carburizing, -nitriding, and – carbonitriding in gas mixtures consisting of H₂, CO or/and NH₃ at temperatures between 920 K and 1120 K and subsequent quenching in brine and liquid nitrogen.