This article explores the motivations behind the loss of a number of Germanic prefixes in the history of English. Using Old and Middle English translations of Boethius’ de Consolatione Philosophiae as a corpus, it is shown that prefix loss is not specific to a single word category, nor to the presence of morphosyntactic characteristics such as prefix separability. This state of affairs cannot be explained by current theories of prefix loss, which are generally restricted to inseparable verbal prefixes. The fact that some prefixes are lost and some are preserved, also argues against an across-the-board grammaticalisation account, based mostly on semantic factors. It is held here that a closer look at the prosodic structure of native prefixes can provide a principled explanation for the entirety of our data. To this effect, the optimisation of a resolved moraic trochee (Dresher & Lahiri, 1991) amid significant restructuring of the language’s lexicon had crucial impact on the fate of prefixed words. In particular, Early Middle English would have come to prefer maximal, branching feet, and avoid words with prefixes constituting heavy, non-branching feet. Ultimately, the preservation of prosodic structure led to the loss of heavy monosyllabic prefixes due to stress clash between prefix and root. Light monosyllabic and bisyllabic prefixes, in contrast, were preserved, since no clash occurred. This argument explains the changes in prefixation from a purely prosodic standpoint, hence accounting for the data for both verbal and nominal prefixes, which were heretofore dealt with separately.