Mixed-layer clays of variable composition and structure occur in core samples from two drillholes (WK207 and WK210) drilled into the Te Mihi sector of the Wairakei geothermal field. These were identified by X-ray diffraction analysis of glycolated and orientated sample fractions at less than 2 um and less than 0.2 um. Low permeability lacustrine sediments encountered by drillhole WK207 contain a well-developed sequence of mixed-layered clays. The shallowest downhole appearance of mixed-layered illite/smectite occurs at 146 m depth where temperature is only 100 degrees C. Discrete illite is present only below 297 m (200 degrees C) in the finer size fraction (less than 0.2 um). Chlorite first appears downhole, in association with illite-smectite, at 177 m depth (110 degrees C). Drillhole WK210 encountered predominantly ignibrites and rhyolites, and fluid flow here is mainly in channel. Within these rocks, a sequence of interlayered clays is poorly developed. Discrete illite and chlorite are present in core from only 224 m (180 degrees C), but the measured temperatures where interlayer clays occur ranges from 140 to 209 degrees C. Differences in the identity of clay minerals present in the Wairakei reservoir, where conditions are otherwise the same, demonstrate the strong control that the type of fluid flow has on their formation. In poorly-permeable sediments, where diffuse fluid flow prevails, a clearly-defined sequence of mixed-layer clays occurs. These are absent where channel flow dominates, the discrete chlorite and illite deposit directly from solution.