Thermogravimetry-mass spectrometry is employed to characterize desorption and decomposition processes that occur when samples containing salicylic and acetylsalicylic acids absorbed to potassium, sodium, and calcium montmorillonites are heated. Whereas salicylic acid exhibits behavior similar to benzoic acid, in that the interlayer cation charge is the most important factor affecting desorption temperatures, acetylsalicylic acid decomposition is significantly affected by montmorillonite water content. When salicylic acid/clay samples are heated, desorption begins at temperatures slightly above 100 degrees C. At higher temperatures, decomposition produces phenol and carbon dioxide. Acetylsalicylic acid does not desorb. Instead, it reacts with water to produce salicylic and acetic acids. For both salicylic and acetylsalicylic acids, decomposition is favored for samples with low absorbate loadings, suggesting that initially absorbed molecules occupy more thermally stable environments than those subsequently incorporated into montmorillonite interlayer spaces. (C) 2016 Elsevier B.V. All rights reserved.