A single-stage subsonic compressor was examined with respect to compressor instabilities. During the inception of rotating stall, the transients of the pressure rise and mass flow were measured as well as their hysteresis. The development of the stall cell and the characteristics of the unstable operating range were determined.Several measurement techniques and data analysis methods for the real-time detection of unstable compressor operation and aspects of their implementation on a digital signal processor are discussed.The results of the work are meant to serve as a basis for the design of an active stall avoidance system.