Municipal mergers are typically contentious and polarizing issues among both citizens and politicians. In deciding on these, municipal-level referendums are often commissioned by municipal councils. Referendums, though, are also per se polarizing processes that only exacerbate an already polarizing issue. Adding deliberation to referendum processes has been shown in previous studies to be a more democratically sustainable process than mere referendums. In this study, we explore the use of mixed deliberation between citizens and politicians within a municipal merger process in the municipality of Korsholm in Finland, one year before a referendum on the issue occurred. The deliberations were two-hour sessions in February 2018, with local politicians present in each discussion group. Using pre- and post-deliberation surveys, we trace how citizens (n = 117) engaging in deliberation developed their social trust, political trust, and political efficacy during deliberation. Generally, we expected that all of these would be strengthened in deliberation. The results, however, reveal only a few statistically significant effects, some of which ran contrary to expectations.