It is widely held that social capital can help build sustainable communities (Cuthill 2010), yet researchers agree that further research is needed to fully understand the social dimensions of sustainable development (Getz 2009). In event settings, understanding how social capital is formed remains limited (Wilks 2011). This paper addresses this issue by examining the extent to which the UK City of Culture 2013 (CoC13) succeeded in building social capital in post-conflict Derry/Londonderry. The authors adopted a case study approach and mixed qualitative methods in the form of in-depth interviews and focus groups. The findings suggest that CoC13 helped to generate both bonding and bridging social capital, however, while intra community bonds were strengthened, exclusivity was also fostered. More positive were the examples of bridging capital cultivated across communities, particularly between young attendees, and between event organisers, with cooperation building trust and goodwill between volunteers. Some of the cross community relationships developed have been sustained. However, the lack of legacy planning has meant that the long-term social goals have not been met. In summary, the study suggests that in a post conflict society, events can help build social capital. However, in the absence of legacy planning, the benefits gained may soon evaporate.