A cubic equation of state (EOS) is used to develop a cohesive technique for the partial molar fugacity coefficient of a component in a mixture. The technique simplifies the determination of fugacity coefficients for any type of mixing rules by the introduction of a differential operator and logarithmic transformation of a mixture property. The technique is very attractive in that it introduces functional modifications into previously established mixing rules of pressure-explicit equations of state, and offers a unique opportunity for a unified fugacity routine in PvT packages and process simulation models. The technique accommodates density-dependent mixing rules as well as non-cubic equations; any type of mixing rule with varying levels of difficulty is admissible by the differential operator, without starting the derivation of mixing fugacity coefficient afresh. Several mixture rules are presented to show the judicous use of the differential operator for the partial fugacity coefficient.