A combining rule is presented that reduces third and higher order virial coefficient-type mixing rules to quadratic. The underlying assumption is that we can determine the contribution of a molecule to the interaction of an aggregate of molecules that includes it by considering how it interacts separately with each of the molecules in the aggregate. The combining rule defines multibody interaction parameters as a linear function of its forming molecules’ pure characterization parameters. The coefficient multiplying each pure characterization parameter is calculated from the contribution of the factors that characterize the interaction of the pairs of molecules that involve the molecule whose contribution to the multibody interaction we want to determine. The new combining rule can be particularly useful in virial type EOSs, but not exclusively, because the composition-dependence of the virial coefficients has the same order as the virial coefficient itself, and use of the combining rule introduced reduces all composition dependencies to quadratic. Use of this combining rule eliminates the need to obtain interaction coefficients for triplets, quadruplets, etc.