A method of positively locating the source of leaks in the heat transfer surface area of the plate heat exchanger between separate product and service fluid paths arranged in an intimitate heat exchange relationship is disclosed. The method comprises: circulating a solution through one fluid path of the exchanger, the solution consisting of solvent with appreciable vapor pressure under the chosen test conditions; providing a gas flow across the other fluid path of the exchange surface by means of forced or natural convection; applying a positive pressure differential between solution and gas stream; and detecting visually the location of deposited solutes on or around the other fluid path.