The remarkable stability of the cooperation among the members of the Organization of Petroleum Exporting Countries (OPEC) has generally been explained by these members’ mutual dependency on high and stable oil revenues. Since the OPEC countries, however, face the double security dilemma of both domestic and external security threats, they are not simply eager to secure (absolute) oil revenues for the sake of domestic stability; they are also sensitive to the (relative) oil revenues of their competing or even conflicting partners. The existing approaches of rational egoism and defensive positionalism have proven to be rather inadequate in explaining this kind of gain-seeking behavior. This paper therefore develops the new theoretical approach of "gain-seeking mentalities," with the objective of tracing variations in OPEC members’ gain-seeking behaviors. Using this approach, the empirical assessment of Iran and Iraq during the Iran-Iraq War and Iraq during the Gulf War of 1990/91 shows the extent to which Iran and Iraq altered their gain-seeking behavior as a result of a changing constellation of threats.