Abstract: An intermeshing passage manifold is a device for subdividing each of two passages of ducts connected to it into multiple adjacent separate intermeshed passages. The intermeshing passage manifold is to be used for heat exchangers and fluid mixing devices in order to minimize pressure (energy) losses and to satisfy geometric constraints in many applications. The intermeshing passage manifold consists of a number of manifold sections adjacent in an end-to-end manner. Each of the manifold sections consists of a number of passage modules adjacent in a side-by-side manner. Each passage module consists of two passages; the orientation of these passages at one end of a module is at right angles to the orientation of the passages at the other end of the module, however, the cross-section shape of the module remains the same throughout its length.