This thesis investigates morphological productivity, which is among one of the most contentious issues in the field of Linguistics (Aronoff, 1976; Plag, 1999; Bauer, 2001). The advent of electronic corpora sparked fresh interest in productivity, notably the highly innovative measures proposed by Baayen and his co-writers (Baayen, 1989; Baayen and Lieber, 1991; Baayen and Renouf, 1996) which advocate the use of hapax legomena (once-occurring words in a corpus). This research re-investigates these methods to pinpoint the degree to which hapaxes represent new words of the language and, therefore, are valid indicators of productivity. The study also develops a more fine-grained method for measuring productivity, based on the re-analysis of the previously-tried methodologies.The case study for this research is negative prefixation. A database of all negatively-prefixed words in the British National Corpus is created and relevant variables, such as types, tokens and hapaxes, are examined to determine which combination provides the most pertinent measure. The thesis also comments on the relative productivity of the negative prefixes and therefore considers appropriate historical and semantic factors.Findings show that though hapaxes do provide access to neologisms, they are not the same phenomenon and cannot, therefore, be seen as a direct reflection of productivity. Significantly, it was discovered that while hapaxes typically include a proportion of neologisms, this proportion varies considerably from affix to affix. The proposed new formulae for measuring productivity - neologism/type and neologism/token - both point towards the prefix non- being the most productive of the five investigated, while in- is the least productive. Findings are also presented about the distribution of the negative prefixes across word class, an area which has never previously been examined in terms of morphological productivity.