Abstract After a long period during which theoretical analyses were presented on evaporative gas turbine cycles, such as the HAT (Humid Air Turbine) and CHAT (Cascaded Humid Air Turbine), the first experimental plant with water recovery is currently under construction at the Lund Institute of Technology in Sweden. The pilot plant is due to start in evaporative mode in May 1998, and this represents the first step for the validation of the humid air turbine concept. One of the main points of interest is the power modulation which should be possible controlling the evaporated water flow rate. If the whole compressed air flow rate is introduced into the evaporator the possibilities to vary its water content are scarce if the temperatures in the recuperator are not changed. A solution to this problem has been patented by Vattenfall AB, and consists in bypassing a fraction of the air entering the evaporator directly into the recuperator. In this paper a detailed study of the different evaporation modes is presented from the point of view of both the first and second law analysis. The thermodynamic analysis will also be compared with the operational flexibility that the by-pass solution offers. Applications to some commercial turbines, which are most suited to use in HAT cycle mode, will also be presented.