Mössbauer spectra of iron-containing Heusler phases yield information on the atomic as well as on the magnetic ordering. The typical regular L21 or inverse Heusler structures provide two distinct crystallographic sites for the iron atoms which can be well distinguished as their isomer shifts and hyperfine fields are different. In Fig. 1 the Mössbauer spectrum of Co1.4Fe1.6Si at room temperature is shown as an example. This compound is derived from the half-metallic ferromagnet Co2FeSi, where cobalt was partially replaced by iron. The spectrum consists of two somewhat broadened hyperfine sextets with hyperfine fields Bhf of 33 and 20 T, respectively. The spectrum confirms that the iron atoms occupy two different crystallographic sites, namely the 8c and the 4a site in the L21 Heusler structure. The different Bhf values reflect different magnetic moments at the two iron sites. On the other hand, Heusler phases which show large atomic disorder, give rise to very broad poorly structured features.