Reasons for unstable behavior of different samples of Kyshtym kaolin during liquefaction are provided. Chemical, phase composition, and microstructure of Kyshtym and Glukhovets kaolin deposits are investigated. It is established that kaolin liquefaction capacity is influenced both by the presence of smectite within kaolin and mainly by the degree of ordering of the kaolinite structure. A larger amount of kaolinite with a highly-ordered structure ensures good kaolin liquefaction and stable properties of casting mixtures based upon them. According to these studies, the possibility of assessing the liquefaction capacity of kaolin based on results of x-ray phase analysis of its structure is established.