This study reports on a corpus analysis of samples of spoken discourse between a group of British and Taiwanese adolescents, with the aim of exploring the statistically significant differences in the use of grammatical categories between the two groups of participants. The key word method extended to a part-of-speech level using the web-based corpus analytical tool, "Wmatrix," highlights those linguistic domains which deserve particular attention. Specifically, it reveals the lexical and grammatical categories that occur unusually frequently or unusually infrequently in the English learners' discourse when compared with the language used by the native speakers of English in the sample. The research findings delineate the pedagogical merit of key domain analysis and thus help to inform English as a foreign language teachers and materials developers in the design of courses emphasising spoken interaction.