Research and literature on teaching adults primarily discusses longer courses within the sphere of formal education, that is formal tertiary education and non-formal education (typically work-place or work-related learning). While both these fields provide a rich source of general information, it is difficult to find research texts that specifically deal with teaching adults in a seminar environment, that is, a planned, one-off learning event ranging in length from three hours to two days. While some research has focused on Thai culture in general and the nature of Thai university teaching in particular, very little has been published concerning the teaching of Thai adults, whether in a formal setting such as a university or college, or in a non-formal, work-place or work-related setting. This research reflects an effort to compensate for this pronounced lack of research in teaching adults in a short course environment and the paucity of research on teaching Thai adults. Using a case-study method, fifteen Western or Western-educated Thai trainers in the Thai short-course market were interviewed to determine the characteristics of an effective short-course trainer instructing courses in English. Based on a semi-structured interview format, with questions framed from the literature review, eight key characteristics were determined that reflect the qualities of an effective trainer in the Thai market. Centered on the vital role played by trainers' knowledge of the participants and their culture, the characteristics identified by the research highlight the need to foreground key aspects of participant culture when planning and presenting a short course in the Thai business community. While content expertise and teaching skills remain important, the key to effectiveness lies in acquiring and skillfully applying a knowledge of participants that goes beyond needs analysis to include a generic knowledge of the social norms that identify Thais as a cultural group throughout the training process. The eight characteristics are discussed in detail, and while some aspects of these are consistent with the conventional wisdom discussed in the literature review, most arose from the analyses of comments provided by the interviewees. This discussion leads into a number of recommendations for new trainers in this market and concludes with insights into further areas of study that could prove useful in Thai and other cultures, and for educators involved in short- or long-course events.