A lightweight, high-efficiency alternating channel counter-flow heat exchanger structure is disclosed. A matrix of alternating hot and cold channels defining a heat exchanger structure is provided. A portion of each of the inlets and outlets of each of the hot and cold channels is blocked to prevent fluid flow through the blocked portion, thus creating hot-only and cold-only fluid communication regions on the ends of the heat exchanger structure. Alternating hot and cold headers provided on each end of the heat exchange structure service the respective hot and cold channels. The partial blocking structures on the channel-ends enable a single hot or cold header/plenum to be offset with respect to individual rows of channels and thus service a pair of adjacent rows of alternating hot and cold channels in the matrix of channels. The true alternating channel counter-flow design provides a higher heat transfer rate than a similarly-sized cross-flow design.