The electrical behavior of iron-doped strontium titanate (Fe:SrTiO3) single crystals equilibrated at 900 degrees C and quenched below 400 degrees C at various oxygen partial pressures (PO2) was investigated via impedance spectroscopy and compared to defect chemistry models. Fe:SrTiO3 annealed and quenched between 1.2x10(-14) and 2.0x10(-4)Pa PO2 exhibits a conduction activation energy (E-A) around 0.6eV, consistent with ionic conduction of oxygen vacancies. However, sudden changes in E-A are found to either side of this range; a transition from 0.6 to 1eV is found in more oxidizing conditions, while a sudden transition to 1.1 and then 0.23eV is found in reducing PO2 These transitions, not described by the widely used canonical model, are consistent with predictions of transitions from ionic to electronic conductivity, based on first principles point defect chemistry simulations. These models demonstrate that activation energies in mixed conductors may not correlate to specific conduction mechanisms, but are determined by the cumulative response of all operative conduction processes and are very sensitive to impurities. A comparison to electrically degraded Fe:SrTiO3 provides insight into the origins of the conductivity activation energies observed in those samples.