The Understanding of volunteers in vaccine trials about their role as study participants and their voluntary commitment during the study are always one of the important concerns apart from evaluation of safety and efficacy of vaccine trials, especially in HIV prophylactic vaccine trials. The apprehension of indirectly risky behavior encouragement and deviated expectations among volunteers should be of concern. The current prospective cohort study aimed to assess and monitor the changes of risk behaviors, attitude and expectations among 164 volunteers from 2 studies of different prophylactic HIV vaccines, the Chiron HIV Thai E gp 120/MF59 +/- the Chiron HIV SF52 gp120 and Aventis Pasteur Live Recombinant ALVAC HIV (vCP1521) priming with VaxGen gp120B/E (AIDSVAX B/E) boosting. 113 males and 51 females with a mean age (+/- SD) of 28.82 +/- 7.97 years old were enrolled from October 1997 to December 1998 and February 2000 to April 2001. Education and risk reduction counseling were regularly performed at every visit and questionnaires about risk behaviors, knowledge, attitudes, social influences and expectations were asked at baseline, 4 months and 12 months. No change of potentially HIV transmission related risk behavior was observed during the studies. There was a statistically significant decrease of risk sexual practices from the beginning of the trials (42.2% vs 1%, p < 0.0001). While 35.2 per cent from 62.2 per cent of the volunteers at the beginning of the study continued sexual practice with an identified single sexual partner at the end of the study (p < 0.0001). All of the volunteers expressed the beneficial expectations as knowledge gain, social contribution, feelings of having gained merit and self-benefits from health check-ups.