X-ray photoelectron spectroscopy (XPS) was used to determine the surface-charge magnitude and distribution for some micas and high-illite I-S clays. Based on predictable layer by layer atomic ratios for margarite, muscovite and sericite it was possible to establish an escape depth of ~15 Å for photoelectrons. After the replacement of surface ions by Ba2+, interlayer ion (Ca, K, Na)/Ba ratios are a measure of the relative layer charges on external surfaces and the first internal interlayer. These ratios can be used to assign charge magnitudes and ascertain layer charge asymmetry when it occurs. The O/Ba ratios determined by XPS serve the same purpose. Diagenetic I-S from the Vienna Basin gives an XPS-determined symmetrical layer charge of 0.41; in a similar clay from the Gulf Coast sequence, the outer surface is smectitic, with an external charge of 0.21, in contrast to an opposing silica layer with a charge of 0.31.

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