The results of MOssbauer and crystal chemical experiments on specimens of silicon‐substituted YIG prepared by solid state reaction indicate that the Fe2+ ion content is substantially lower than the Si4+ content resulting in large concentrations of cation vacancies. These vacancies can occur in all three types of cation sites. It is highly probable that some of the small number of Fe2+ ions present are in dodecahedral sites.The results of MOssbauer and crystal chemical experiments on specimens of silicon‐substituted YIG prepared by solid state reaction indicate that the Fe2+ ion content is substantially lower than the Si4+ content resulting in large concentrations of cation vacancies. These vacancies can occur in all three types of cation sites. It is highly probable that some of the small number of Fe2+ ions present are in dodecahedral sites.