So-called category-determining prefixes in English (befool, delouse, disbar, encage, out- jockey, unsaddle) have been treated as exceptions to the Righthand Head Rule (Williams 1981). This article argues that so-called category-determining prefixation is a V (Verb)-to-V prefixation which takes denominal and deadjectival converted verbs as inputs, and thus special treatment is unwarranted. The hypothesis that conversion underlies N (Noun)/A (Adjective)-to-V prefixation is examined from both diachronic and synchronic perspectives. Diachronically, it is shown that the prefixes in question all started as non-category-determining V-to-V prefixes, and their N/A-to-V usage was established only in Modern English. With the constant productivity of conversion in the history of English, N/A-to-V usage can emerge from V-to-V usage. Synchronically, denominal/deadjectival prefixed verbs are shown to exhibit input and output properties that prove the above hypothesis: they have a converted counterpart; they are subject to the same morphological constraints as converted verbs; and their semantics is equivalent to the semantics of converted verbs modified by the semantics of V-to-V prefixation. It is concluded that there is no derivational prefix that determines the output category in English.