Using game-theoretical modelling, this paper analyses the role of risk perception and signalling on HIV transmission between commercial sex workers (SW) and their clients. The paper also provides results of a survey conducted in late-2014 in Thailand involving 200 SWs (female sex workers, FSW, and male and transgender sex workers, MTSW), as well as 67 clients regarding condom use and HIV testing. It was found that: (1) incomplete information and individual risk perceptions are important factors for engaging in unprotected sex; (2) there is potential for signalling but it is rather weak in practice; and (3) MTSW, as well as men, who have sex with men (MSM) represented the highest-risk groups that are highly vulnerable and exposed to HIV.