Chemical diffusion coefficients have been reliably measured in the case of the electronic conductor SrTiO3 (Fe-doped) and the ionic conductor yttria-stabilized ZrO2 (Ni-doped) by using a contactless optical relaxation technique. A quantitative concept (proposed by one of us earlier) which allows the inclusion of internal trapping effects leads to a consistent analysis of (D) over tilde in terms of temperature (T), oxygen partial pressure (P) and doping level (m). In both materials, SrTiO3 and ZrO2, the argument between experiment and theory is very good. In the first case, due to the availability of all the necessary parameters, no adjusting parameter has been used. In both cases, the analysis also leads to a reevaluation of literature data and is able to reconcile the apparent "scatter" of published results.