Research on computer-mediated communication (CMC) has moved from the linguistic aspect to the sociolinguistic dimension in order to understand how human linguistic behaviour is influenced by the availability of new technology. Given that technological development has enabled the representation of many more non-Romance languages on the internet, computer users of these languages can communicate in a greater range of language choice. The current study examines the influence of computer technology on orthographic code and language choice in Instant Messaging (IM) among fluent bilinguals in Hong Kong, drawing on data from a corpus of IM dialogues as well as comments from telephone interviews and email discussions with IM users. Code-switching models including the Matrix Language Frame Model (Myers-Scotton, 1993a, 2002; Myers-Scotton & Jake, 1995) and Gafaranga and Torras’ (2001) proposal for the notion of medium are applied to analyze bilingual discourse in online interaction. It is found that the diversity of Cantonese orthographic codes is chiefly explained by technological availability but is also affected by other linguistic and functional factors. A changing practice of CMC is also observed through the study of code-switching patterns which not only includes the emergence of a composite Matrix Frame but also changes from a dominant use of English to the adoption of a bilingual medium involving both Cantonese and English. Future research based on the new set of bilingual data contributed by technological improvement will be valuable to the understanding of many linguistic and sociolinguistic phenomena such as language change and language contact.