AbstractThis work considers the importance of vapor‐phase nonideal mixing in high‐pressure equilibrium and rate‐process calculations. New techniques are presented for calculating vapor‐phase fugacity coefficients in nonideal mixtures.The equilibrium properties of gaseous mixtures are represented by a virial equation of state which is adequate for the vast majority of chemical engineering problems. Techniques based on an extension of the corresponding‐states theory are given for estimating the various coefficients appearing in the equation of state; these estimates appear to be reliable for a large variety of nonpolar or weakly polar systems. Techniques based on the theory of intermolecular forces are also presented for polar systems and for a limited class of mixtures containing hydrogen‐bonding components. The proposed methods of calculation are illustrated by a large variety of problems occurring in common chemical engineering calculations, and it is shown that failure to correct for vapor‐phase nonideal mixing can, in some cases, lead to very