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This paper describes a new method of thermodynamic study of the gas turbine cycle, with emphasis to the second law analysis. In the first part a general method is given for calculating the exergy of the combustion air, of the fuel, of the air/fuel mixture and the combustion gas, in addition to their enthalpy and entropy. In a second part of the paper a 'Unified Diagram' is compiled for facilitating the first as well as the second law analysis of the cycle. In the final part the basic thermodynamic processes of the gas turbine are critically analysed from a first as well as a second law point of view. The 'Unified Diagram' is designed for a 'typical liquid fuel' of a hydrocarbon nature, certain ambient conditions (pressure and temperature) as well as a given air/fuel ratio. The values entropy, enthalpy and exergy of the combustion air, of the fuel/air mixture as well as of the combustion gas can be read from the 'Unified Diagram' and all thermodynamic processes of a gas turbine cycle can be easily illustrated on this diagram. Several such large diagrams have been compiled, corresponding to different air/fuel ratios. All these diagrams have been compiled for combustion gas temperatures up to 1600 deg K (1327 deg C). As basic ambient values have been taken 1 bar and 298 K (25 deg C). The use of the diagrams is illustrated by 1 example, a sample of a complete such diagram is included in the paper.