The thermal transformation of Ba exchanged zeolite X to celsian has been studied by Al and Si MAS NMR spectroscopy. Evidence for the degradation of the zeolite framework is present in the Si NMR spectra after thermal treatment at 850 °C. Confirmation is provided by the Si NMR data that synthesis of celsian via the decomposition of Ba exchanged zeolite leads to a single defect phase. Clustering of the isomorphous replacement of aluminium by silicon must occur to explain the observed Si chemical shifts. The Al NMR data show distorted aluminium co-ordination sites upon the thermal transformation of Ba exchanged zeolite X. The distortions present in the amorphous matrix are greater than those present in the monoclinic and hexagonal crystalline phases of celsian.