The efficiency of a process of compression or expansion of a gas is commonly defined in terms of the change in energy which occurs as compared with the change required in isentropic flow. In another method the efficiency may be defined in terms of the fraction of the mechanical work lost in friction and converted in to heat. Alternatively, if the process is adiabatic, the efficiency may be defined in terms of the fraction of the enthalpy increment which is re-converted into heat by the frictional effects. This latter method is applied here to adiabatic subsonic expansion of a gas in steady flow and some simple relationships of a general nature are established. The application to simplified flow through a turbine nozzle is them considered with particular reference to the choking mass flow.