Ultrafine M-type hexagonal ferrites of nominal composition Sr1-xLaxFe12-x ZnxO19 and Sr1-xLaxFe12-xCoxO19 with x = 0, 0.1, 0.2, 0.3 and 0.4 were produced by chemical co-precipitation. The phase make-up of the samples was investigated by x-ray diffraction. The analyses show that the hexagonal M-type phase is the main phase in all the samples. Secondary (La, Sr)FeO3, ZnFe2O4 and CoFe2O4 phases are also detected, indicating that the La, Zn and Co content in the M-type phase is lower than the nominal content. A complete Mossbauer analysis of the substitution effects in the M-type phase was made. The results show that the M-type phase contains La3+ in the same proportion as Zn2+ and La3+ in the same proportion as Co2+, in the La-Zn and La-Co samples, respectively. The evolution with x of the hyperfine parameters of the components used to fit the contribution of the M-type phase have been interpreted consistently in relation to the substitution effects by means of the comparison between the spectra of the La-Zn and La-Co substituted samples. The results show unambiguously that La 3, ions are located in the Sr2+ sites, Zn2+ ions are located in the 4f(1) sites and Co2+ ions are located in both 4f(2) and 2a sites.