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A standard thermogravimetric analysis instrument was modified by simple changes of the gas flow lines and by provisions for continuous ultradrying, to afford higher working sensitivity and eliminate undesired mass gains due to the presence of residual impurities, especially moisture, in the sample chamber. The modified instrument is, in particular, useful in gravimetric gas sorption studies of hygroscopic materials and in thermochemical decomposition of ultrasmall (<100 μg) salt samples. To illustrate the advantage of this apparatus, two case studies are presented. In the first, gas adsorption has been measured at room temperature over zeolite molecular sieves after their high-temperature activation; in the absence of moisture readsorption during cooling, “true” N2 and Ar uptake values have been obtained and the difference between N2 and Ar adsorption at room temperature has been shown to probe the zeolitic charge density. In the second study, accurate decomposition analysis of basic magnesium carbonate has been performed on small samples and successfully accomplished even with a ∼80-μg sample; comparison of the analyses of samples of different size indicates that the decomposition mechanism of this salt may be mass-size-dependent.