Spatial separation of activator oxides in MnO₂ + PbO and MnO₂ + V₂O₅ binary compositions helps to locate the interactions between the oxides that lead to nonlinear joint effects of the compositions on GaAs thermal oxidation. The mutual enhancement of the chemostimulating activity (a positive nonlinear effect) was discovered in the solid phase, whereas vapor-phase processes led to a considerable negative nonadditivity of the joint chemostimulating effect.