Conversion is a word-formation process characterised, among other aspects, by the formal identity between the original word and the resulting one (bottle à to bottle). However, there are a few cases of conversion-related words which challenge this formal identity characteristic of conversion. One of these cases is that of noun-verb conversion pairs such as importN-importV, in which a type of phonological base allomorphy occurs: stress shift. In such cases, stress shift consists in nouns tending to be stressed on the first syllable (importN /ˈɪmpɔːt/) while verbs are usually stressed on the last one (importV /ɪmˈpɔːt/). This study aims to determine which are the most frequently occurring noun-verb conversion pairs displaying stress shift and why this type of allomorphy occurs. To answer these questions, a corpus of 157 noun-verb conversion pairs was compiled from frequency lists of nouns and verbs. Out of these pairs, 25 presented stress shift. Additionally, information about the etymology and the year of introduction into English of the 25 pairs with stress shift was gathered. It was found that all the noun-verb conversion pairs with stress shift are of Romance origin. Furthermore, the results suggest that the stress shift in noun-verb conversion pairs might be due to their adaptation to the Germanic stress system after being introduced into English from either Latin or French.