The morphological and prosodic classification of the two German negative prefixes in-and un-has generally been based on the prefixes’ behavior with regard to nasal place assimilation: in-is said to assimilate to the following plosive’s place of articulation, while un-has been claimed to retain its alveolar nasal. The study presented in this paper investigated whether this long-standing claim can indeed be confirmed empirically via a production experiment that compared F2 trajectories of German in-and un-sequences followed by either an alveolar or a velar plosive: a) between words; b) at morpheme boundaries (i.e., as a prefix); c) within single morphemes. Results showed that un-does undergo nasal place assimilation as a prefix, in stark contrast to previous claims in the literature. Furthermore, a clear difference was found between the three contexts, with strongest assimilation patterns in the within-morpheme condition, weaker assimilation at the morpheme boundary, and no assimilation between words. This paper thus demonstrates the importance of empirical experimentation for the formulation of phonological generalizations.