Community mobilisation strategies for HIV/AIDS prevention based on recognition of social vulnerability and concepts of empowerment have emerged at the forefront of international efforts to reduce the AIDS pandemic, increasingly replacing a focus on individual risk. This paper describes the start-up phase of a participatory learning and action project to create a sense of community identity as a first step towards collective action among some 300 young, debt-bonded, brothel-based migrant sex workers from Vietnam in Phnom Penh, Cambodia. The factors that make sex workers vulnerable to HIV also pose considerable barriers to mobilising them, due to competing interests between and among brothel owners and sex workers. Discussion and visual tools--e.g. a spider diagram of causes of unsafe sex and how to overcome these--are being used in group work to analyse concerns expressed by sex workers, along with survey questionnaires and in-depth interviews. In the second phase the project will address sensitive topics such as violence and unsafe sex in more depth, in hopes of protecting the emerging solidarity among sex workers and shifting the balance towards greater co-operation.