Abstract: A method for the detection of leaks in a heat exchanger having discrete first and second fluid flow paths is disclosed. The method comprising the steps of introducing a contaminant fluid into the first flow path (13) and testing for the presence of the contaminant fluid in the second flow path (14), wherein the pressure in the second flow path (14) is maintained below atmospheric pressure. The pressure in the second flow path may be maintained, for example, at between 0.1 bar (1×104 N/m 2) and 1 bar (1×105 N/m 2) below atmospheric pressure. Apparatus for carrying out the method is also disclosed.