Results of a series of experiments show that impact of acidization on clays increases with decreasing pH and increasing temperature. Experiments using acidic buffers with pH values ranging from 1.0 to 5.0 were performed to simulate the laboratory removal of carbonates from calcareous soils. Impact of acidization was measured by relative changes in X-ray diffraction (XRD) peaks of three clays (a smectite, an illite, and a kaolinite). Results show that dissolution of carbonates at pH < 5.0 causes serious changes in XRD peaks, as does dissolution at temperatures over 40°C. Buffers with high buffer capacities also cause changes in XRD responses, although these can be corrected by uniform saturation of interlayers. Results of experiments using pH = 5 buffers are similar to those using various forms of EDTA. The results indicate that removal of carbonates from calcareous soils with buffered acids for careful analysis of clays should be done at relatively high pH at room temperatures.