The question of which water enters the Indonesian passages is a subject of outstanding debate. This article represents another attempt to address this issue using a new nonlinear model. The new model illustrates that the origin and composition of the Indonesian throughflow are determined by the structure of the opposing and retroflecting currents situated to the east of the passages. The nonlinear “layer-and-a-half model” is composed of an eastern and western basin (corresponding to the Pacific and Indian Oceans) connected via a channel representing the Indonesian passages. The channel cuts through a separating wall which runs from the northwest to the southeast; the retroflecting currents are taken to be zonal. Nonlinear solutions are constructed analytically by balancing the flow force associated with the boundary currents flowing along the wall in the western Pacific. It is found that, without retroflection (that is, the entire flow along the western wall in the Pacific enters the passage), the throughflow must originate in both hemispheres, with 70% of the transport coming from the north and the remainder coming from the south. It is impossible for the flow to originate only from one side of the channel's entrance because, under such conditions, the momentum imparted by this flow on the fluid near the entrance to the channel cannot be balanced. When retroflection is present (that is, only a fraction of the western boundary current system in the Pacific enters the channel) and the coastline is tilted, the above division of transports is dramatically altered. For some range of parameters the balance requires that fluid exit the channel rather than enter it. This is not the case, however, for the Indonesian throughflow, where about 1 Sv must enter from the south and 11 Sv (±5 Sv) must enter from the north. Since opposing retroflecting currents flowing next to a solid wall containing no channels or gaps can be stationary only if their transports satisfy a given ratio, it is suggested that the fact that the retroflection is situated immediately to the east of the Indonesian passages is not accidental. Namely, it is argued that the western boundary current system in the Pacific is stationary because it is situated next to the Indonesian passages. In this particular position, the portions of the transports which do not allow satisfaction of the stationarity condition leak out into the Indian Ocean.