Objectives: The aims of this study were to estimate household demand in the general population of Thailand for a (hypothetical) preventive HIV vaccine; to determine whether spouses in the same household would purchase the same number of vaccines for household members and have the same demand function; to determine whether spouses would allocate vaccines to the same household members; and to estimate household and per capita average willingness to pay (WTP) for an HIV vaccine price. Methods: The data come from a national contingent valuation survey of 2524 residents (aged 18-20 years) of 1235 households in Thailand during the period 2000 to 2001. In a subsample of 561 households, both head of household and spouse completed independent (separate) interviews. Respondents were asked whether they would purchase an HIV vaccine for themselves and for other household members if one were available at a specified price. Results: For the full sample, average household WTP for the vaccine was substantial (US$610 at 50% vaccine effectiveness, US$671 at 95% effectiveness); the average per capita WTP for household members was US$220 at 50% effectiveness and US$242 at 95% effectiveness. Although spouses reported that they would purchase the same total number of vaccines, and had essentially the same demand functions, at lower vaccine prices wives were significantly more likely than husbands to allocate vaccines to their daughters than to sons. Conclusions: Because wives are more likely to allocate vaccines to daughters, vaccination programs aimed at women and girls might have different outcomes than programs directed at males or at all potential adults without regard to sex.