For the system of SrTi1-xAlxO3-x/2-delta(x=0-018), the thermodynamic factor (|partial derivative|n a(O2)/ partial derivative delta| )of component oxygen was evaluated against oxygen activity in the range of 10(-8) <= a(O2)<= 1 from the as-measured oxygen nonstoichiometry (6) isotherms at elevated temperatures of 800 degrees o 1000 degrees C (Part 1). The factor would normally show a single maximum [at the close vicinity of the stoichiometric point (delta=0)] in its near stoichiometry region (delta approximate to 0) or 2[V (center dot center dot)(O)] approximate to [Al-Ti('),]. It has been found that the factor unusually exhibits two maxima due to the internal redox equilibrium of variable-valent background impurities or the like which are higher in concentration than intrinsic electrons at delta=0. Oxygen chemical diffusivity was measured against a(O2), by a conductivity relaxation technique in the same activity range at temperatures of 8000 to I 100 1C. It appears to vary convex-upwardly up to the order of magnitude of 10(-3) cm(2)/s. This trend of variation as well as magnitude are satisfactorily explained in terms of the unusual thermodynamic factor and electronic transference number in accord with the classic Wagner theory of chemical diffusion. (c) 2007 Elsevier B.V. All rights reserved.