Modelling of turbochargers based on steady state operating maps for turbine and compressor rotors is widely recognized to have limitations arising from flow pulsations and heat transfer effects that are not well accounted for by interpolation and extrapolation from the mapped data. With implementation of low pressure exhaust gas recirculation systems and multi-stage boosting, the inlet conditions of centrifugal compressors vary more widely than traditional single stage compression systems. Understanding the impact of the inlet conditions on irreversibilities, and therefore efficiency complements existing works on pulse flow and heat transfer effects in informing the engine modelling community.This research experimentally explores the effect of inlet pressure and temperature on the total-total efficiency of a steady flow centrifugal compressor across a range of conditions in isolation of pulse flow effects and with negligible heat transfer. The results suggest that for any given corrected mass flow rate and total-total pressure ratio the isentropic efficiency is dependent on inlet conditions. The effect of a 40 K change in inlet temperature or a 0.5 bar change in inlet pressure results in up to 15 % point +/− 1% point change in total-total efficiency at low rotor speeds and up to 5 % points +/− 1% point at higher rotor speeds. Particular emphasis has been placed on quantifying uncertainties and repeatability. The effect of inlet temperature and pressure is shown to be dependent on the location within the map. The paper concludes with a discussion of existing loss model forms and their ability to represent the effects observed.