Analyses of natural minerals suggest the wide variety of both cationic and anionic substitutions possible in the pyrochlore structure. Limits on the possibilities of ionic substitution in pyrochlore have been determined by the preparation of a large number of compounds. An important feature found is that substitution of up to two of seven anions is possible but apparently no more. Many ion‐deficient structures can be made, especially those with one large cation absent. Anion deficiency to the same extent or even in smaller amounts leads to the formation of perovskite structures. Quantitative data on solid solubility among three related structures, pyrochlore, perovskite, and columbite, have been determined for the case of Cd2Nb2O7‐NaNbO3‐CdNb2O6. An attempt has been made to study the relation of pyrochlore and C‐type rare earth to the fluorite structure as an example of the ordering of