One important aspect in the design of social protection is coverage. In Peru, as in most Latin American countries, social security coverage is mandatory only for workers in the formal sector. This article investigates the determinants of voluntary affiliation to Peru's individual accounts pension system. It is found that married males with more than high-school level education living in high income households and with other family members already affiliated to the individual accounts pension system have a higher likelihood of voluntary affiliation. Although the results suggest that family-based safety nets might be substitutes for voluntary participation in the individual accounts pension system, nonetheless, extending pension coverage and addressing poverty remain as challenges for government involvement. © 2009 The author(s). Journal compilation © 2009 International Social Security Association.