A series of tin + antimony mixed oxide powders, calcined at 773 K, has been studied by electrical conductivity. A continuity in the electrical behaviour is found between semiconducting SnO₂ and insulating Sb₂O₄. Pure stannic oxide is an n-type semiconductor and its free electrons come from the first ionization of anionic vacancies whose concentration is ≈ 10¹⁸ cm^–3 at 608 K under 2.13 × 10⁴ Pa O₂. The enthalpy of formation of these vacancies and the ionization energy of their 2nd electrons have been estimated. As the Sb content increases, antimony dissolves into the SnO₂ structure in the 5⁺ state, which increases the conductivity, σ, up to a maximum corresponding to 6.1 Sb atom %. Around this value formation of the Sb₂O₄ phase begins. The increased conductivity of mixed oxides with high Sb content (>20 atom %), compared with that of Sb₂O₄, which is an insulator, is attributed to a doping effect by Sn⁴⁺ cations in Sb³⁺ lattice positions of Sb₂O₄. Comparison of how both σ and catalytic properties vary with Sb content shows that electron transfer between catalyst and adsorbed species is not the rate limiting step in propene oxidation and that the solid solution of Sb⁵⁺ in SnO₂ cannot constitute the active phase for acrolein formation.