The mixing device is constructed with a coarse mixing zone and fine mixing zone. The two zones are each generally constructed with corrugated plates or the like which are layered together to define a plurality of inclined criss-crossing flow ducts. The ducts in each layer are at least partially open to each other to allow cross-mixing. However, the upstream, or coarse, mixing zone is provided with a plate-like intermediate element, or the like, between each pair of adjacent corrugated plates to close off the adjacent flow ducts in the middle zones of the respective layers while leaving the edge zones open at least partially. This latter construction is to avoid a concentration gradient over the mixing zone and to provide a uniform distribution of the flowing media at the exit end of the zone.