The clay mineralogy of over ninety samples of Old Red Sandstone sedimentary and volcanic rocks from various parts of Scotland has been investigated by X-ray diffraction, differential thermal analysis, infrared spectroscopy and electron and optical microscopy. The Downtonian stage of the Lower Old Red Sandstone is characterized by kaolinitic clays, mainly of detrital origin. Overlying Dittonian rocks contain little kaolinite and frequently yield a variety of interstratified minerals including chlorite-vermiculite, dioctahedral smectite-chlorite and illite-smectite, largely from fresh and altered volcanic debris. In the Middle Old Red Sandstone of Caithness the dominant clay minerals are illite and chlorite derived from erosion of micaceous and chloritic rocks in the source area. Samples from the Middle Old Red Sandstone of other areas, and from the Upper Old Red Sandstone, contain mainly kaolinite and interstratified illite-smectite often of diagenetic origin. Differential thermal analysis and electron microscopy show that there are two distinct types of illite-smectite. The first is characterized by normal temperatures of dehydroxylation (530-550 degrees C.) and by an indeterminate platy morphology; the second dehydroxylates at abnormally high temperatures (670-680 degrees C.) and occurs in slender, well-developed laths. The dominant iron oxide mineral is hematite, with minor goethite, and electron microscope observations indicate that both minerals, in part at least, crystallized after sediment deposition.