The physicochemical changes in the solid phases obtained during the hydrothermal crystallization of LTL zeolite in pure potassium form at 423 K have been reported. Samples of varying crystallinity, obtained at different crystallization periods, were characterized by different techniques such as XRD, DTA/TG, MAS-NMR, IR, chemical compositions, and the sorption data. The significant changes observed in the framework IR spectra (400-1200 cm(-1)) were found to be helpful in estimating the progress in the crystallization. The area under the low temperature endotherm in DTA was also correlated with percent crystallinity of the sample. Except in the lower range, XRD crystallinity was in excellent agreement with IR crystallinity and with the crystallinity evaluated from the benzene uptake. The crystallinity evaluated from the uptake of water, n-hexane, cyclohexane, and nitrogen showed nearly sigmoid type curves indicating the progressive crystallization. Both Si-29 and Al-27 MAS-NMR spectra showed the changes in the number of different silicon environments and existence of all the aluminum species in the ordered tetrahedral coordination, respectively. The changes observed in the chemical composition of the samples of different crystallinity were found to be useful in understanding the crystallization mechanism.