Catalytically active tin–antimony oxides have been investigated by 121Sb Mössbauer spectroscopy. Materials prepared by coprecipitation and dehydrated at 600 °C for 16 h give single-phased blue solids containing both SbIII and SbV in oxygen environments. Materials containing more than 20% antimony and heated for longer periods or at higher temperatures give the segregation of an additional antimony oxide phase. Samples with an antimony concentration of less than ca. 10% contain SbV within the tin(IV) oxide rutile lattice. Although the Mössbauer spectra provide evidence for an increase in s-electron density at both tin and antimony nuclei, the data give no indication of any reduction processes leading to the formation of either SnII or SbIII. A mechanism for charge compensation is proposed which involves the delocalization of electrons into a conduction band. The reaction of the tin–antimony oxides with hydrogen and the use of the material in the catalytic oxidation of olefins is accompanied by the reduction of SbV to SbIII.