Oriented slides of twelve different mixtures of kaolinitic and illitic underclays were studied with the X-ray spectrometer and the resulting curves were evaluated on the basis of the area of the 001 peaks. Although the areas from the unmixed components varied greatly, within any given mixture the two clays were quantitatively comparable on the basis of a 1/1 001 peak area ratio. The considerable peak area variations from slide to slide are the result of different degrees of preferred orientation and are not due to any appreciable differences in crystallinity. Mixed-layering of expandable material with the illitic component does not affect the quantitative comparisons significantly.