This paper outlines a novel meanline off-design model to predict the performance characteristics of a radial inflow turbine that operates with ideal and real working fluids. Experimental data available in open literature were used for validation; including radial turbines that operate with both ideal gas (air) and real working fluids (R123). Initially the differences in the expansion process on a thermodynamic base between ideal and real fluids are highlighted. Then, the proposed meanline off-design model is calibrated for a few selected points and validated against experimental data for both air and R123. The comparison between the predicted and measured results presented errors less than 10% for both ideal and real gas fluids. Finally, the predicted air turbine was simulated with a real gas fluid. Relative to air, operation with R123 revealed that the peak efficiency is 12% lower and occurs at 70% lower rotational speed. The proposed methodology gives insights for accurate model based design of organic Rankine cycle systems, as the radial turbo expander is the most crucial and expensive component of such heat recovery systems.