A plate-fin heat exchanger core comprising: a plurality of fin layers arranged in a stack, each fin layer defining a fluid flow channel, the fin layers comprising first alternating fin layers defining a fluid flow channel in a first direction and second alternating fin layers arranged to alternate with the first fin layers in the stack, defining a fluid flow channel in a second, different direction, and a plurality of closure bars, each closure bar having: a substantially rectangular main body portion defined by a first edge and a second edge and an end portion having a first end portion edge, and opposite second end portion edge and an end edge extending between the first end portion edge and the second end portion edge, wherein the first edge of the main body portion and the first end portion edge form a continuous substantially straight first closure bar edge and wherein the second end portion edge is spaced from the first end portion edge by a distance greater than the distance between the first edge and the second edge of the main body portion, and wherein the second edge of the main body portion and the second end portion edge joined by a radius portion define a second edge of the closure bar; the core including a first plurality of said closure bars arranged to seal the first alternating fin layers on a first side of the stack and a second plurality of said closure bars arranged to seal the second plurality of fin layers on a second side of the stack adjacent the first, characterised in that: the first closure bars are arranged such that their end portions overlap and their end edges align, and the second closure bars are arranged such that their end portions overlap and their end edges align, and wherein the end portions of the first and second closure bars overlap to define a solid corner of the stack, and wherein the first closure bars are stacked in an order such that topmost and bottommost closure bars have a second end portion edge of a first length and the length of the other first closure bars in the stack decrease with respect to the first length towards the middle of the stack to form a curved inner end portion profile from top to bottom of the stack. the core including a first plurality of said closure bars arranged to seal the first alternating fin layers on a first side of the stack and a second plurality of said closure bars arranged to seal the second plurality of fin layers on a second side of the stack adjacent the first, characterised in that: the first closure bars are arranged such that their end portions overlap and their end edges align, and the second closure bars are arranged such that their end portions overlap and their end edges align, and wherein the end portions of the first and second closure bars overlap to define a solid corner of the stack, and wherein the first closure bars are stacked in an order such that topmost and bottommost closure bars have a second end portion edge of a first length and the length of the other first closure bars in the stack decrease with respect to the first length towards the middle of the stack to form a curved inner end portion profile from top to bottom of the stack. the first closure bars are arranged such that their end portions overlap and their end edges align, and the second closure bars are arranged such that their end portions overlap and their end edges align, and wherein the end portions of the first and second closure bars overlap to define a solid corner of the stack, and wherein the first closure bars are stacked in an order such that topmost and bottommost closure bars have a second end portion edge of a first length and the length of the other first closure bars in the stack decrease with respect to the first length towards the middle of the stack to form a curved inner end portion profile from top to bottom of the stack.