AbstractA correlation is given for the prediction of third virial coefficients of pure substances and mixtures in terms of second virial coefficients. The substance properties which are needed for the correlation are the critical temperature, the reduced dipole moment, and the parameters for the hard body modelling the molecule. The correlation gives the third virial coefficient C in the temperature interval 0.85 < T/Tc < 1.5 within the experimental accuracy. For lower temperatures the correlation predicts positive values of C throughout, in contradiction to the current opinion, and this is made plausible by additional arguments. Values of C of polar components are highly sensitive on the reduced dipole moment. Mixture values of C are in similar agreement to experiment as pure component values.