The isentropic exponent is one of the most important properties affecting gas dynamics. Nonetheless, its effect on turbine performance is not well known. The paper discusses a series of experimental and computational studies to determine the effect of isentropic exponent on the flow field within a turbine vane. Experiments are performed using a newly modified transient wind tunnel which enables annular cascade testing with a wide range of working fluids and operating conditions. For the present study tests are undertaken using air, CO2, R134a and argon, giving a range of isentropic exponent from 1.08-1.67. Measurements include detailed wall static pressures which are compared with computational simulations. Our results show that over the range of isentropic exponents tested here, the loss can vary by between 20%-35%, depending on vane exit Mach number. The results are important for future turbines operating with real-gas effects and/or those where high gas temperatures can lead to variations in isentropic exponent.