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The Chemical Index of Alteration (CIA) was expressed as CIA (molar) to become a more sensitive measure of the degree of chemical weathering. The CIA (molar) has the value of 1 for fresh feldspars and for unweathered, non-peraluminous igneous rocks, but increases towards infinity as chemical weathering progresses. The utility of this index as a humidity indicator for both modern and ancient muds was tested. A database comprising 281 entries with chemical data for modern and ancient sediments deposited under arid, semi-arid, semi-humid and humid climate was assembled from the literature, and used to calibrate the relationship between humidity and CIA (molar) values. For both modern and ancient sediments, the histograms display a shift towards higher values with increasing geological indicators of humidity, whereas sediments deposited under conditions inferred to be arid from other geological indicators show values tightly clustered around 1. The Permian of the Parana Basin was then used as an example of the applicability of the CIA (molar) for paleoclimatic studies. Major element analyses (Al(2)O(3), K(2)O, Na(2)O, CaO) were performed on 55 claystones and shales from three drill cores in a stratigraphic succession spanning the Early to Late Permian. The CIA (molar) for these rocks accorded with expectations based on sedimentologic and paleontologic evidence, discriminating well between arid and humid conditions in the basin independently determined from sedimentological and paleontological indicators. The use of the CIA (molar) as a paleo-humidity indicator, however, can be limited by the presence of carbonate-rich sediments, the occurrence of post-depositional K(+) addition (metasomatism, metamorphism, diagenetic illitization), as well as by inheritance of clays from sedimentary rocks in the source area. Nevertheless, with appropriate caution, the CIA (molar) is a useful tool for the assessment of the humidity conditions in the rock record. (C) 2010 Elsevier B.V. All rights reserved.