Humidification of the flow through a gas turbine has been proposed in a variety of forms. These include the so-called evaporative gas turbine (EGT) cycle, in which water is injected in the compressor discharge in what is essentially a regenerative gas turbine cycle. In another variation water is also added downstream of the evaporative aftercooler, continuously in the heat exchanger. The operation of heat exchangers under these conditions (of "cold stream" water injection) is discussed: work on heat exchanger effectiveness is first briefly reviewed; and previous analysis of how the thermal capacity of the cold gas stream can be increased by water injection is developed, for both reversible and irreversible flows. It is shown how heat exchanger performance is modified, and the amount of water to be injected is determined. Finally the thermal and rational efficiencies of evaporative gas turbine cycles are discussed, using both first and second law analysis, with relevance to the results on heat exchangers described earlier.