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The human urinary calculi are mainly constituted by calcium oxalate, magnesium ammonium phosphate hexahydrate, and uric acid. The ions or molecules are easily characterized by wet chemical methods. The difficulties appear in the differentiation of the hydrates of calcium oxalate (monohydrate COM or Whewellite, and dihydrate COD or Weddelite). A high level of COD in the urinary stones leads, often, inflammation, sharp pain and blood in urine. In the worse cases, they must be extracted by surgical way. The identification of the main components of urinary calculi, the knowledge of the true number of water molecules bounded to the calcium oxalate, and the determination of each hydrate in the mixture, are the interests of this memory. The thermal analysis (simultaneous DTA-TG) was applied on thirty-three urinary calculi. The determination of the calcium oxalate hydrates was confirmed by calorimetry (DSC).