In this paper we attempt to determine the criteria speakers apply in morphologically or syntactically ambiguous cases. In German separable vs. inseparable verbs can be included in these cases. Although most grammar books present patterns or rules which give the impression of relatively clear cut cases, in contrast literal usage in every day life seems to hint to the contrary. In an empirical study with 120 students we demonstrated that separable verbs are a grammatical entity where even native speakers differ greatly in their linguistic behaviour and choices. Their behaviour also reveals considerable incongruities with rules ascribed by grammarians. In addition we found the following factors which influence the way speakers use complex verbs: While stress plays a major role (as pointed out in most grammar books), habitual use or analogous treatment enter as important factors. Semantic criteria are not heeded or only to a small degree, contrary to the postulations in some grammar books. Our results support the hypothesis that for syntactical processing, criteria or cues from various linguistic levels (phonological, morphological, semantic) are used. Since those cues can be contradictory to one another, insecure or differing linguistic judgements ensue. Thus we gained insights into how syntactic knowledge is stored and processed.