In this two-part paper, the phenomenon of part span rotating stall is studied. The objective is to improve understanding of the physics by which stable and persistent rotating stall occurs within high speed axial flow compressors. This phenomenon is studied both experimentally (Part I) and through the use of unsteady RANS simulations (Part II). In this paper, the behavior of an eight stage high speed compressor is studied during slow acceleration maneuvres along a fixed working line. Casing mounted pressure transducers and rotor mounted strain gages are used to examine the spectral content of any unsteadiness in the flow and its behavior across the operating range. By deliberate aerodynamic mismatching of the front stages through adjustment of three rows of variable stator vanes (VSVs), stable rotating stall is initiated. The observed behavior falls into two “families” of high and low frequency when tracked on the instrumentation. Further analysis based on the Doppler shift between the static and rotating measurements confirms that these respective phenomena are due to rotating stall of high and low cell count. Acoustic modes resulting from stall/rotor interaction are also identified. Strong correlation of the stall intensity with simple 1D meanline predicted loading parameters suggests that these families of behavior are independently linked to the stalling of different regions within the compressor.