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M-type Sr 1 La Fe 12 Co O 19 ferrite powders with = 0 to 0.4 were investigated using Mössbauer spectrometry in the 77 K-550 K temperature range. The Mössbauer spectra were fitted by assuming that Co 2+ is present in both 4f 2 and 2a octahedral sites, and that some Fe 2+ is present in the 2a site. On increasing , the main changes concern the Mössbauer contributions of the 12k, 4f 2 and 2b sites. These changes are interpreted in relation with the Co 2+ -Fe 3+ substitution at the 4f 2 site, the Fe 3+ -Fe 2+ valence change at the 2a site and the La 3+ -Sr 2+ substitution. The temperature dependance of the hyperfine field at the Fe sites of the Sr 0 6 La 0 4 Fe 11 6 Co 0 4 O 19 compound is similar to that of the unsubstituted compound. However, the curvature of the temperature dependence of the hyperfine field at the 12k site slightly increases with . This reflects the increase of the curvature of the temperature dependence of the magnetization of the substituted compounds.