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The harmonic vibrational force fields and the IR spectrum of XSO 2NCO (X=F,Cl) molecules have been studied using ab initio HF/SCF method with the 6 31G * basis set. The ab initio harmonic force fields are scaled empirically using the scaled quantum mechanical (SQM) method of Pulay. A set of scale factors are optimized by the least squares fitting to the experimental frequencies of FSO 2NCO and then are transferred to ClSO 2NCO to give an a priori prediction of its fundamental frequencies. The average deviations between the theoretical frequencies and the experimental values for FSO 2NCO and ClSO 2NCO are 3 and 5 cm -1 , respectively. The assignments of the fundamentals for these two molecules are also made according to the potential energy distributions and the ab initio IR intensities.