The effects of humid air on the performance of a multistage research compressor and new methods of humidity accounting to ensure appropriate representation of performance parameters are investigated in this paper. Turbomachinery textbooks present methods of correcting speed and mass flow rate using perfect gas assumptions, but these methods can reduce the ability to achieve repeatable compressor performance when using unconditioned air in a climate where absolute humidity may vary. Instead, a new method is introduced, which models humid air as a real gas and circumvents the need for assumptions in the correction process. In the area of compressor research, the ability to measure small changes in performance parameters and ensure repeatable results is essential. Errors of more than 0.5% can result from using perfect gas assumptions to calculate corrected speed, which can lead to misrepresented performance parameters beyond the uncertainty of the measurements. Multiplicative correction factors based on analytical data are also introduced as an alternate method of applying the new real-gas method, and these correction factors are compared to those derived by previous authors applying ideal gas methods for humidity accounting. This is the first time in open literature that experimental results for a component of a gas turbine engine are presented comparing a humid air correction method with traditional correction methods.