Ruthenium-99 Mössbauer spectra have been obtained for the ternary oxides SrRuO₃, CaRuO₃, BaRuO₃, and Y₂Ru₂O₇ and for the quaternary oxides Sr(Ru_1–xIr_x)O₃(x= 0·1 and 0·2) and Sr(Ru_0·7Mn_0·3)O₃. The spectrum of SrRuO₃ reveals the first example of a hyperfine magnetic field in a ruthenium compound; the internal field of 352 kG at 4·2 K is compatible with the ferromagnetic moment derived from neutron-diffraction data, and confirms the collective-electron magnetism model for this perovskite. A more precise value for the E2/M1 mixing ratio of 2·72 ± 0·17 has been obtained for the ⁹⁹Ru Mössbauer γ transition. The oxide CaRuO₃ shows only a single line and hence no magnetic ordering even at 4·2 K; it is not antiferromagnetic as previously reported but is Curie-Weiss paramagnetic. The oxide BaRuO₃ also gives a singlet; there is no long-range antiferromagnetic ordering, but within each cluster of three face-sharing RuO₆ units localised pairing of the 4d electrons takes place. The oxide Y₂Ru₂O₇ is magnetically ordered at 4·2 K, with a hyperfine field of 126 kG. Substitution of ruthenium by iridium in the oxide SrRuO₃ reduces the field at ruthenium by approximately 60 kG for each iridium nearest neighbour, whereas in the case of manganese substitution the reduction for each manganese neighbour is only 22 kG.