Experiments have been performed to determine the heat-transfer characteristics for each plate of a two-plate colinear array aligned parallel to the flow direction and situated in an airflow in a flat rectangular duct. The pressure drop associated with the plates was also determined. The plate thickness and the interplate spacing were parametrically varied (for a fixed plate length), as was the Reynolds number. It was found that while the Nusselt number may either increase or decrease with plate thickness, depending on the operating conditions, it should increase for conditions corresponding to those commonly encountered in an interrupted-plate heat exchanger. The pressure drop increase caused by increasing plate thickness is greater than the largest thickness-related increase of the Nusselt number. The presence of the inter-plate gap affects the Nusselt numbers for both the first and second plates, but to a greater extent for the latter. In many cases, the curves of Nusselt number vs spacing attain a local maximum and indicate that the conventional spacing-to-length ratio of unity is not necessarily optimal.