Honeycomb body interdigitating mixers are disclosed along with methods for producing them, the honeycomb body having parallel cells extending along a common direction, the mixer comprising a first fluid path extending within the honeycomb body along a first path direction perpendicular to the common direction, the first fluid path defined within a first plurality of said cells, at least a high aspect ratio portion of the first fluid path having an aspect ratio of height in the common direction to width perpendicular to the common direction and to the first path direction of at least 3:1, and a second fluid path extending within the honeycomb body along a second path direction perpendicular to the common direction, the second fluid path defined within a second plurality of said cells, at least a high aspect ratio portion of the second fluid path having an aspect ratio of height in the common direction to width perpendicular to the common direction and to the second path direction of at least 3:1, wherein the first fluid path is fluidically connected to the second fluid path within their respective high aspect ratio portions via a group of apertures, the group extending in the common direction, the group of apertures taken together having a ratio of height in the common direction to width perpendicular to the common direction of at least 3:1.