A procedure was developed for the calculation of gas turbine cycles using as fuels, a general hydrocarbon or Hydrogen. Computer programs were developed, to facilitate calculations, by suitably modifying the existing ones, developed by the National Research Council, Ottawa. Using these programs, the effect of changes in the fuel composition and heat of combustion on the performance of turbojet, turbofan,-turboprop and turboshaft cycles was studied. The performance of different fuels was compared with the standard fuel performance. On the basis of this study, the ranges of H/C ratio and heat of combustion of the fuel, within which the 'standard fuel approach' is reliable, was found. A simple modification of the approach was expected to be reliable within a wider range of fuel properties.