Loneliness is a frequent negative feeling among older people. A programme aimed at alleviating loneliness among older people by promoting social capital, i.e. social support and participation, was conducted in primary health care centres in Spain. We aimed to explore participants’ experiences of loneliness and social participation before the programme, perceived programme effects and contextual influences. A descriptive-interpretative qualitative design was used. 41 persons were included comprising older people, health and social care professionals, and volunteers. Data were collected through three focus groups, 36 semi-structured interviews and participant-observation of the intervention. A thematic content analysis was applied. Older persons with diverse profiles of loneliness and participation decreased their loneliness, increased their knowledge and participation in local community assets, and developed companionship, a sense of belonging, peer support and friendship. Their mental wellbeing increased and participants could deal better with health or family problems. An empowerment process was observed. However, loneliness persisted among some widowed participants and health and social vulnerabilities hampered some impacts. Conflicts and exclusion were occasional unintended effects. The promotion of social capital in ageing to alleviate loneliness involves complex processes interrelated with health and socio-economic factors. Future programmes should be adapted to local contexts and participants’ characteristics.