Mixed dielectric (Bε12) and pressure (Bp12) second virial coefficients of CF3H–Ar and CFH3–Ar mixtures of 50 °C have been obtained by cyclic expansion and low pressure dielectric measurements. These give Bε12=85 cm6/mole2 for CF3H–Ar and 2 cm6/mole2 for CFH3–Ar, the former being much larger and the latter much less than calculated for spherical polar molecule interactions and Lennard–Jones 12–6 central force parameters derived from Bp12 and interaction viscosity η12. The differences can be accounted for by values of the Buckingham and Pople shape factor of D=−0.18 for CF3H and +0.32 for CFH3; these values are in reasonable agreement with results of Copeland and Cole for the pure polar gases and with estimates from molecular models by Barnes and Sutton.