Fundamental frequencies of SiH2F2 and their isotopic shifts were calculated using ab initio MP2 and QCISD(T) and DFT (B3LYP) methods. The accuracy of calculations was tested by comparison with experimental SiH2F2 and SiD2F2 frequencies. While most of the fundamental frequencies are predicted very well, the accuracy of calculations of the deuterium isotopic shift was found to be 20–40cm−1. The most noticeable shift on silicone isotope substitution was found to be ∼7cm−1 for the Si–F stretch mode of B2 symmetry. Some doubts upon the accuracy of the quantum chemical calculations of the silicone isotopic shifts induced us to find a more accurate method to calculate isotopic shifts. Therefore, the method of analytical calculation of isotopic shifts for the B1 and B2 vibration modes of dihaloid silanes was proposed and tested. The results of analytical calculations of the isotopic shifts of the Si–F and Si–Cl stretching vibrations of SiH2F2 and SiH2Cl2 molecules under isotopic substitution of silicone and chlorine atoms are reported.