This chapter reports a first study on perceptual discrimination of speech quality change along separate perceptual quality dimensions (“discontinuity,” “noisiness,” and “coloration”), as induced by different types of speech quality impairment (random frame loss, signal-correlated noise, and band-pass filtering). It is demonstrated that listening subjects are able to distinguish between these quality impairment types and associated perceptual dimensions, in case average perceived degradation intensities are kept constant. Arguably, distinct internal references are formed that represent these quality dimensions.