The difference in intensities of the 10 A X-ray diffraction maxima of sedimented clay minerals after treatment with ethylene glycol and after heating is generally assumed to be due to the collapse of expandable clay minerals to 10 A. Therefore, this difference is commonly used in the calculation of the proportion of these expandable clay minerals. To determine the effects of both temperature and time at elevated temperature on this collapse, we used three clay materials, a shale, a glauconite, and a synthetic mica-montmorillonite containing illite and mixed-layer illite-smectite but no discrete smectite. The < 2 mu m fraction of each material was sedimented on glass slides to make at least 96 "identical" samples of each. Our results are as follows: (1) The optimum temperature of collapse of illite-smectite mixed-layer clay minerals to 10 A in the materials that we tested was about 375 degrees C. (2) Maximum collapse of the expandable smectite layers to 10 A was complete after 30 minutes. (3) After an initial heating interval of about 5 minutes, temperature proved more critical than time. (4) To obtain maximum peak intensity for heated clay minerals, diffraction intensity of the 10 A peak must be measured while the sample is still above 100 degrees C. Cooling in a desiccator is not an adequate alternative. (5) Even when the greatest care is taken to produce samples as similar as possible, considerable variation can be expected in the 10 A peak intensity. We believe, therefore, that when one uses oriented or sedimented clay samples, the data should not be reported to more than one significant figure.