Hydrogen ion conductivity in 2% and 10% Al-substituted SrTiO3 has been investigated by transport number measurements using the concentration cell/emf method in wet atmospheres as a function of pO(2) (10 (- 20) -1 atm) and temperature (350 -1000 degreesC). Earlier indications of apparent negative charge transport by hydrogen under reducing conditions and high temperatures have been confirmed. By the present measurements, possible artefacts from the type of acceptor-dopant, gas buffer, electrode material, and porosity of the sample appear to have been ruled out. Electrochemical pumping experiments with gas chromatography were inconclusive with respect to hydride ion transport. Thermogravimetry as a function of hydrogen activity did not show evidence of hydride ion incorporation, but indicated uptake of neutral hydrogen under reducing conditions and high temperatures. Quantum molecular dynamics simulations indicate the existence of defect species or clusters that may be reminiscent of interstitial hydrogen with a tendency to associate with effective negative charge, e.g., on neighbouring titanium ions, under simulated reducing conditions.