The thermal transformation of Ba- and (Ba + Li)-exchanged precursors of zeolites A and X into monoclinic celsian has been investigated by X-ray diffractometry, differential thermal analysis, and thermodilatometry. Upon the proper thermal treatment either Ba-exchanged zeolite A or X give rise to the following transformations: zeolite --> amorphous phase --> hexacelsian --> monoclinic celsian. Zeolite A-based precursors are more reactive than zeolite X-based precursors allowing all the reported transformations to occur at far lower temperatures starting from Ba-exchanged zeolite A rather than from Ba-exchanged zeolite X. In particular, starting from zeolite X-based precursors, fully monoclinic celsian may be obtained after 24 h at 1550 degrees C in the absence of Li and after 6 h at 1100 degrees C in the presence of Li, whereas starting from zeolite A-based precursors it may be obtained after 6 h at 1100 degrees C in the absence of Li and by slow heating (1 degrees C/min) up to 900 degrees C in the presence of Li.