Abstract: Leakage is detected between a process fluid and a temperature-conditioning fluid, or from a process fluid to a temperature-conditioning fluid, in an industrial process. The industrial process includes an A and a B fluid, and one of the A and B fluids receives heat from or transfer heat to the other of the A and the B fluids by an indirect contact method, and one but not both of the A and the B fluids is an industrial process fluid. At least one specie of tracer chemical is maintained in the A fluid, and that specie of tracer chemical is not a normal component of the B fluid. At least one of the A and the B fluids is subjected to at least one analysis at least one site. Such analysis at least detects the presence of the specie of tracer chemical when the fluid subjected to the analysis is the B fluid, and such analysis at least determines the concentration of the specie of tracer chemical when the fluid subjected to the analysis is the A fluid.