Honeycomb body interdigitating mixers have parallel cells extending along a common direction, the mixer comprising a first fluid path extending within a honeycomb body perpendicular to the common direction, 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 perpendicular to the common direction, the second fluid also having a high aspect ratio portion having an aspect ratio 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 to width of at least 3:1.