BACKGROUND:In high-income countries such as Australia, an increasing proportion of HIV cases have been acquired overseas, including among expatriates and travelers. Australia's national strategies have highlighted the need for public health interventions for priority populations. One approach is to expand efforts to places or spaces where expatriate communities reside. Online settings such as forums used by expatriates and travelers have potential for preventing sexually transmissible infections with those hard to reach through more traditional interventions. OBJECTIVE:Our objectives were to (1) identify and describe domains of social interaction and engagement in 1 online forum used by Australian expatriates and travelers living or working in Thailand; and (2) make recommendations to health-promoting organizations and policy makers regarding the role of these forums in public health interventions with mobile populations who may be at risk of acquiring HIV or other sexually transmissible infections. METHODS:We identified forums and users in 2 stages. We identified 13 online forums and analyzed them for inclusion criteria. We searched 1 forum that met the required criteria for users who met inclusion criteria (n=5). Discussion threads, rather than individual posts, were units of analysis. For each user, we collected as transcripts the first 100 posts and 10 most recent posts, including the thread in which they were posted. We analyzed and thematically coded each post (n=550). Transcripts and analyses were reviewed and refined by multiple members of the research team to improve rigor. Themes were not totally emergent but explored against symbolic interactionism concepts of presentation of self, meaning, and socialization. RESULTS:Key domains were as follows: the forum (characteristics of the space and reasons for use), gaining access (forum hierarchy and rules), identity (presentation of self and role of language), advice, support, and information (sources of information, support provided, influencers, topics of discussion, and receptiveness to advice), and risk (expectations and perceptions). The forum exhibited evidence of unique language, rules and norms, and processes for managing conflict and key influencers. The forum was a substantial source of health information and advice provided to users via confirmation, reassurance, or affirmation of beliefs and experiences. Risk perception and expectations varied. Risk taking, including around sex, appeared to be a key expectation of travel or the experience of being an expatriate or traveler. CONCLUSIONS:Australian expatriate and long-term traveler participation in the online forum formed, influenced, and reinforced knowledge, attitudes, interaction, and identity. Such forums can be used by policy makers and health-promoting organizations to provide supplementary sources of support and information to hard-to-reach mobile populations who may be at risk of acquiring HIV or other sexually transmissible infections. This will complement existing engagement with health professionals and other public health interventions.