Test results pertaining to the characteristics of single-stage centrifugal compressors with backswept impellers and channel-type diffusers are presented and analyzed to formulate major performance criteria influencing maximum diffusion capability. For any given stage, it was determined that stage surge (when triggered by diffuser stall), occurred near a constant mean stream velocity diffusion ratio between the impeller tip and diffuser throat. This diffusion ratio attained a maximum value of 1.8 for impeller tip Mach numbers less than unity, but was not unique for all stages, being more intimately coupled with throat blockage accumulation as a function of diffusion rate. This was identified by testing some vaned diffusers beyond the stall limit where rapid blockage accumulation precipitated an immediate decrease in channel diffuser and system static pressure recovery. The results of various experiments in the vaneless space are also described to illustrate the sensitivity of the vaneless space flow upon centrifugal compressor performance.