In a plate heat exchanger having a welded plate pair comprising first and second plates (2, 4) which are welded together at a contact region (19), a by-pass area (22) is defined between the plates on the inboard side of the contact region (19). The plates (2,4) are so shaped and arranged that a second by-pass area (22a) similar to the first, is defined between the second plate (4) and the first plate (2a) of an adjacent, similar, plate pair, on the inboard side of a gasket (28) clamped between the two plate pairs. A gasket groove (18) in the first plate has an inner side wall which is substantially continuous and which is of fixed or variable height substantially greater than zero.