In this investigation samples of exhaustively Ba-exchanged zeolites Ba–A, Ba–X, and Ba–LSX were subjected to thermal treatments in the temperature range 200–1550 °C for times up to 40 h. Their thermal transformations were comparatively studied by room temperature X-ray diffraction. Upon heating, the three zeolites undergo the following sequence of transformations: zeolite --> amorphous phase --> hexacelsian --> monoclinic celsian in strongly different ways. In particular it was found that: (1) The crystallisation of nuclei of monoclinic celsian dispersed in the amorphous phase arising from the thermal collapse of the structure of zeolite Ba–A, recorded above 500 °C in a previous work, did not occur for zeolites Ba–X and Ba–LSX. (2) Zeolite Ba–A exhibited a far greater reactivity than zeolites Ba–X and Ba–LSX; actually, the conversion of hexacelsian into monoclinic celsian attained completion at 1300 °C, starting from zeolite Ba–A, whereas did not attain completion in the most severe experimental conditions experienced (1550 °C, 24 h), starting from zeolites Ba–X and Ba–LSX. Such different thermal behaviours could be reasonably ascribed to the different middle-range order occurring in the amorphous phases arising from the thermal collapse of the microporous structures of zeolites Ba–A, Ba–X, and Ba–LSX.