Mobile populations, including truck drivers, are at elevated risk of acquiring HIV and other sexually transmitted infections (STI). However, measures of mobility have been poorly operationalized and little research exists exploring the psychosocial context of mobility and sexual risk taking. A systematic sample of 1775 male truck drivers underwent interview at two customs stations on the Southern Brazilian international border in 2003. The psychosocial effect of being mobile was assessed by clustering truckers based on perceptions of the liminal environment, or being outside of one's normal social environment. The relationship between physical mobility (nights spent at home) and liminal cluster with sexual partnerships was assessed. The clustering procedure yielded three dispositions towards the liminal environment. Compared to truckers in the baseline cluster, those who perceive the environment as (1) very, or (2) moderately permissive had increased odds of reporting a commercial sex partner in the past six months and reported increased numbers of commercial partners. For each week slept at home, the odds of reporting a commercial partner decreased by a factor of 0.73 and the average number of commercial partners decreased by a rate of 0.76. Physical and psychosocial measures of mobility were associated independently with increased partnering on the road. Additional exploration of how the liminal environment shapes mobile populations' sexual decision making and vulnerability to STI is warranted.