Mean dipole polarizabilities α 0(λ, T) as well as second optical (or refractive index) virial coefficients b R(λ, T) and second density virial coefficients B(T) of gaseous CH3OH and CCl2F2 have been determined by precise measurements of the refractive index n(λ, T, p) [543 nm ⩽ λ ⩽ 633 nm, 300 K ⩽ T⩽ 355 K, p<0.25 bar (CH3OH) and p<3 bar (CCl2F2)]. α 0 critically compared with the few data in literature. The b R of these gases was measured for the first time with the cyclic-expansion method. The values of ¦B¦ and b R=3160(25) cm3 · mol−1 measured for CH3OH are considerably greater than the values calculated by Buckingham's statistical-mechanical expressions for a Stockmayer interaction potential. This difference is discussed by assuming dimerization via H bonds, with result ΔH 2 0 ≈−(28 ... 33) kJ · mol−1 and ΔS 2 0 ≈−(116 ⋯ 133) J · mol−1 · K−1 for the dimerization enthalpy and entropy for standard conditions, respectively. On the other hand, Buckingham's formulae can be used with success to estimate b R and B of CCl2F2.