A central concern of linguistic phonetics is to define criteria to determine the phonological status of sounds or sound properties observed in phonetic surface form. Based on acoustic measurements we show that the occurrence of schwa in German is determined exclusively by segmental and prosodic structure, with no paradigm uniformity effects. We argue that these findings are consistent with a uniform representation of syllabic sonorants as schwa sonorant sequences in the lexicon. The stability of schwa in CVC-suffixes (e.g. German diminutive suffix -chen), as opposed to its phonetic absence in segmentally comparable context, is argued to be conditioned by the prosodic organisation of such suffixes external to the phonological word of the stem.