Neighborhood characteristics such as racial composition and social capital have been widely linked to health outcomes, but the direction of the relationship between these characteristics and health of minority populations is controversial. Given this uncertainty, we examined the relationship between neighborhood racial composition, social capital, and black all-cause mortality between 1997 and 2000 in 68 Philadelphia neighborhoods. Data from the U.S. Census, the Philadelphia Health Management Corporation's 2004 Southeast Pennsylvania Community Health Survey, and city vital statistics were linked by census tract and then aggregated into neighborhoods, which served as the unit of analysis. Neighborhood social capital was measured by a summative score of respondent assessments of: the livability of their community, the likelihood of neighbors helping one another, their sense of belonging, and the trustworthiness of their neighbors. After adjustment for the sociodemographic characteristics of neighborhood residents, black age-adjusted all-cause mortality was significantly higher in neighborhoods that had lower proportion of black residents. Neighborhood social capital was also associated with lower black mortality, with the strongest relationship seen for neighborhoods in the top half of social capital scores. There was a significant interaction between racial composition and social capital, so that the effect of social capital on mortality was greatest in neighborhoods with a higher proportion of black residents and the effect of racial composition was greatest in neighborhoods with high social capital. These results demonstrate that age-adjusted all-cause black mortality is lowest in mostly black neighborhoods with high levels of social capital in Philadelphia.