BACKGROUND: In many developing countries, initiatives are underway to strengthen voluntary community based health insurance as a means of expanding access to affordable care among the informal sector. However, increasing coverage with voluntary health insurance in low income settings can prove challenging. There are limited studies on determinants of enrolling in these schemes using mixed methods. This study aims to shed light on the characteristics of those joining a community health fund, a type of community based health insurance, in Tanzania and the reasons for their membership and subsequent drop out using mixed methods. METHODS: A cross sectional survey of households in four rural districts was conducted in 2008, covering a total of 1,225 (524 members of CHF and 701 non-insured) households and 7,959 individuals. In addition, 12 focus group discussions were carried out with CHF members, non-scheme members and members of health facility governing committees in two rural districts. Logistic regression was used to assess the determinants of CHF membership while thematic analysis was done to analyse qualitative data. RESULTS: The quantitative analysis revealed that the three middle income quintiles were more likely to enrol in the CHF than the poorest and the richest. CHF member households were more likely to be large, and headed by a male than uninsured households from the same areas. The qualitative data supported the finding that the poor rather than the poorest were more likely to join as were large families and of greater risk of illness, with disabilities or persons with chronic diseases. Households with elderly members or children under-five years were also more likely to enrol. Poor understanding of risk pooling deterred people from joining the scheme and was the main reason for not renewing membership. On the supply side, poor quality of public care services, the limited benefit package and a lack of provider choice were the main factors for low enrolment. CONCLUSIONS: Determinants of CHF membership are diverse and improving the quality of health services and expanding the benefit package should be prioritised to expand voluntary health insurance coverage.