A heat exchanger core which comprises: a) first and second groups of interleaved plates (11, 12) which are arranged respectively to carry first and second heat exchange fluids (F 1, F 2), the plates (11, 12) being bonded to one another and each of the plates (11, 12) in each group being formed in at least one of its faces with at least three, separate, groups of parallel channels each connected to a common inflow and common outflow port, each group of parallel channels forming a platelet (17, 20), b) ports (18, 21) extending through the first and second groups of plates (11, 12) for conveying the first and second heat exchange fluids (F 1, F 2) to and from the platelets (17, 20), and c) distribution channels (19, 22) connecting opposite ends of each platelet (17, 20) in each of the plates (11, 12) to associated ones of the ports (18, 21) located at opposite ends of each of the platelets (17, 20), the distribution channels that are associated with each of the platelets in the plates of the first group being disposed such that they cross without communication with the distribution channels (19, 22) that are associated with respective ones of the platelets (17, 20) in the plates (11, 12) of the second group whereby each one of the platelets in the plates of the first group is located in heat exchange juxtaposition with a respective one of the platelets in the plates of the second group, characterized in that the group of parallel channels of which each of the platelets (17, 20) is composed extends in a direction transversely across the platelet (17, 20) containing plate (11, 12), the ports (18, 21) are arranged in arrays of fluid inflow and fluid outflow ports such that one of an array of fluid inflow ports and one of an array of fluid outflow ports is located at opposite ends of each of the platelets (17, 20), and each of the parallel channels of each of the platelets (17, 20) is formed to provide a tortuous path for heat exchange fluid. a) first and second groups of interleaved plates (11, 12) which are arranged respectively to carry first and second heat exchange fluids (F 1, F 2), the plates (11, 12) being bonded to one another and each of the plates (11, 12) in each group being formed in at least one of its faces with at least three, separate, groups of parallel channels each connected to a common inflow and common outflow port, each group of parallel channels forming a platelet (17, 20), b) ports (18, 21) extending through the first and second groups of plates (11, 12) for conveying the first and second heat exchange fluids (F 1, F 2) to and from the platelets (17, 20), and c) distribution channels (19, 22) connecting opposite ends of each platelet (17, 20) in each of the plates (11, 12) to associated ones of the ports (18, 21) located at opposite ends of each of the platelets (17, 20), the distribution channels that are associated with each of the platelets in the plates of the first group being disposed such that they cross without communication with the distribution channels (19, 22) that are associated with respective ones of the platelets (17, 20) in the plates (11, 12) of the second group whereby each one of the platelets in the plates of the first group is located in heat exchange juxtaposition with a respective one of the platelets in the plates of the second group,