Isotropic atom-atom potentials for use in liquid simulations of 1,1,1,2-tetrafluoroethane (HFC-134a; CF3CH2F) and pentafluoroethane (HFC-125; CF3CHF2) are constructed using effective interactions. These semirigid molecules have interaction centres at the atomic sites and internal rotation about the C-C bond. Intermolecular interactions consist of repulsion-dispersion and Coulombic parts. The internal rotation about the C-C bond is described by a small angle cosine Fourier expansion in the dihedral angle. The potential functions are adjusted to give simulated liquid properties for temperatures between 0.5 (T/T-C) and 0.9 (T/T-C) on the saturated liquid curves. The proposed potentials are used in constant pressure-constant temperature and constant volume-constant temperature molecular dynamics simulations of saturated liquid HFC-134a and HFC-125. Simulated results for thermodynamic, structural and dynamic properties are presented, and compared with experimental data where available.