Abstract
Chemical reactions of 
and 
have been analyzed using chemical potential diagrams for the La‐Zr‐M‐O systems under a condition of 
. A 
vs. 
plot is appropriate for representing diffusion paths, because the layered arrangement of the reaction zone corresponds well to the geometrical configuration of stability polygons of reactants (
and 
) and products (
, MO, 
, etc.). A complicated arrangement including a ternary perovskite phase, 
, observed in the 
couple can be represented by a simple diffusion path having the same slope in the chemcial potential diagram as in the 
couple. The effect of lanthanum nonstoichiometry for 
has been discussed in relation to 
formation and manganese dissolution into YSZ cubic phases.