This article presents the first results of chlorine residual salt analysis (Cl-RSA), a new technique that allows investigation of the isotopic composition of chlorine in oil-field waters from core analysis. As water evaporates, the residual salts precipitate in the pores of a core. Unlike the routinely used strontium isotope residual salt analysis (Sr-RSA) technique, which determines the composition of a trace element (strontium), the Cl-RSA technique allows the determination of the isotopic composition and origin of one of the most abundant anions in natural waters. The results show that combining the ³⁷Cl of two successive leachates of a core sample leads to an isotopic composition that is representative of pore water. The reproducibility of the method applied to sandstone samples is considered to be 0.05. The Cl-RSA technique was tested on two wells from the Elgin area (North Sea, United Kingdom). The ³⁷Cl results in formation waters range from 1.03 to 0.57, with an overall increase with depth. These results have been compared to the ⁸⁷Sr/⁸⁶Sr ratios of pore waters determined in the same wells using the Sr-RSA technique. The similarity of form between the ³⁷Cl and ⁸⁷Sr/⁸⁶Sr profiles with depth confirms that residual salts permit a reliable investigation of the chlorine isotopic composition from core analysis. As with the Sr-RSA technique, Cl-RSA could be used to test the isotopic homogeneity of a sample and to assess the compartmentalization of oil fields and reservoirs. This technique could provide key information regarding the origin of the salinity and its evolution during the reservoirs' filling, as well as help in the monitoring during field production.