The effect of frequency (100 Hz to 1 MHz) both on complex permittivity and on complex impedance data was used to study the conduction processes in Sn-Sb-O systems between 300 and 873 K. The conductance of tin oxide was increased by the addition of antimony oxide at all frequencies and a peak in conductivity was observed at 5 wt % Sb2O3. The effect of frequency on conductance was in agreement with the universal behaviour of solid dielectrics, whilst the atmosphere-dependent capacitance indicated surface dipole phenomena. The catalytic activity and selectivity patterns for the oxidation of propylene above 650 K may be correlated with the observed electrical behaviour, which is related to the mobility of lattice oxygen in the outer layers of the surface.