Thermal dehydration of [Y-6(mu 6-O)(mu(3)-OH)(8)(H2O)(12)(NO3)(6)](NO3)(2).4H(2)O (1), simplified as Y-6(OH)(8)(NO3)(8) . 15H(2)O (1a) or Y(OH)(1.67)(NO3)(1.33). 2.5H(2)O (1b) occurs in five separate steps. The difference of the dehydration process in moist and dry air in the 25-220 degrees C range was investigated by simultaneous TG-DTA. Four hydrates with an approximate composition Y-6(OH)(8)(NO3)(8) . yH(2)O (y=12.5 (2), 10 (3), 8.5 (4), 6 (5)) were isolated by isothermal TG in moist atmosphere at specified temperatures and characterised by FT-IR and XRD. The reversibility of dehydration of the compounds against moist air during storage at room temperature is described. The fully dehydrated product with a good stability in the 140-380 degrees C range was found to be Y(OH)(x)(NO3)(3-x)(x=1.67) (6), instead of a compound with x=1.5 (6a), as had been supposed and described in a previous paper [1]. An alternative way of evaluating the TG-curves, which overcame discrepancies between observed and calculated mass losses, is described.