Hydrothermal synthesis experiments were conducted to study the transition from smectite to corrensite. A mixture of oxides with the bulk composition of corrensite--Na (sub 0.4) (Si (sub 6.4) Al (sub 1.6) )(Mg (sub 7.8) Al (sub 1.2) ) - O 20 (OH) 10 --was sealed in platinum capsules with 29-37 wt. % water. One set of samples was treated in cold-seal vessels at 500 degrees C and 2kbar for durations of 2, 3, 6, 12, and 24 h; the other set was treated at 350 degrees C and 2kbar for periods of 12 to 89 d. X-ray diffraction patterns (XRD) of oriented aggregates from treated products were obtained from ethylene glycol-solvated and air-dried preparations. Samples were also heated to 350 degrees C either in a calibrated muffle furnace, removed and quickly places in a nitrogen filled chamber on the diffractometer, or were heated at 350 degrees C by using a calibrated heating stage mounted on the diffractometer. Initial mineral assemblages at both temperatures contained only saponite and serpentine. In experiments at 500 degrees C, saponite transformed to corrensite within 6 h; in experiments at 350 degrees C, the transformation occurred as early as 22 d. Increased experiment times at both temperatures produced increasing amounts of well-crystallized corrensite, as indicated by several well-defined XRD peaks. No evidence of randomly interstratified chlorite-smectite (C-S) precursor to corrensite was found. The identification of pure smectite, as opposed to highly-expanded randomly interstratified C-S, was possible only when clays were dehydrated on a heating stage on the diffractometer. These results call for a new examination of hydrothermally-altered basalt that has been reported to contain randomly interstratified C-S as an intermediate step in the reaction of smectite to corrensite or chlorite. These results also strengthen the view held by increasing numbers of investigators that corrensite should be regarded as a single phase, not as a mixed-layered silicate.