This is a second cut about the expansion of international organizations following the 2005 paper on cooperation between IOs. Cooperation may vary in depth and extent, because IOs opt for various balances between autonomy and interdependence. Some modalities of cooperation are limited in time and space; others are multi-stakes, multilevel and enduring. Cooperation depends on one major set of explanatory variable: in their expansion, IOs enlarge their initial mandate, which in turn produces overlap between their activities. This in turn compels them to opt for defensive or offensive strategies over boundary issues; and more or less intensive cooperative endeavors.