Abstract: XRFS with sample preparation by pressure-moulding was applied to the determination of chlorine in samples of soil and river sediment. A set of CRM's was selected to prepare the calibration curve, and values of detection limit and RSD (n=10) were found to be 4.2 μg·g^-1 and 5.3% respectively. Furthermore, a study on the law and cause of variation of the determined values of chlorine contents in various types of soil and sediment samples as changed with the increase of frequency of replicate determinations, was made by applying the proposed method. It was shown that for samples of low chlorine contents, the determined values of chlorine contents were found to be increased with the increase of frequency of replicate determinations; and for samples with medium chlorine contents, values of chlorine content determined were kept relatively unchanged or raised only slightly; and for samples with high chlorine contents, the chlorine contents determined in replicate determinations were found lowered to a slight extents. A brief discussion and preliminary explanation for this phenomenon were given.