Oxygen nonstoichiometry (6) was measured on the system of SrTi1-xAlxO(3-x/2)-delta,(x=0.018) as a function of oxygen activity in the range of 10(-18) <= Po-2,/atm <= 1 at elevated temperatures of 800 degrees to 1000 degrees C by a coulometric titration technique with a resolution of - 10 ppm. It has been found that in the stoichiometric regime (delta approximate to 0) where oxygen vacancy concentration is essentially fixed by the majority dopant concentration (2[V-O] :approximate to: [Al-Ti(')]), the isothermal variation of nonstoichiometry deviates anomalously (apparently with three inflections) from the usual, sinehyperbolic trend with a single inflection. This is interpreted as being due to a valence change of variable-valent, background impurities or the like in the minority, and their amount (- 80 ppm) estimated from the deviation. Subsequently, the related defect chemical parameters, including the equilibrium constants for the ionization of the unknown variable-valent entities and the hole trapping by the majority dopants AI(Ti)('), are evaluated. (c) 2007 Elsevier B.V. All rights reserved.