Abstract. The M6ssbauer fractionsffor various ferrousand/or ferric-containing oxides and oxyhydroxides, silicates and carbonates were evaluated from the experimental temperature dependence of their center shifts, using the Debye approximation for the second-order Doppler shift. It is concluded that ferrous ions exhibit a lower fraction as compared to ferric ions. Using standard mixtures of e-F%O3 with selected Fe z + or Fe 3 + compounds, it is found that the calculated Fe 3 + fvalues are somewhat overestimated with respect to those of Fe 2 +. Possible explanations for this shortcoming are discussed and it is suggested that a different temperature dependence of the intrinsic isomer shift is the most likely reason. This suggestion is corroborated by analyses of hematite and hedenbergite data which are available for temperatures up to 900 K and 800 K respectively.