Heats of formation for cyclic C4F8 and C4F8+ are determined at the G3MP2 lever. The several decomposition pathways are investigated. The calculations confirm that C4F8+ rearranges and its decomposition is responsible for both the C2F4+ and C3F5+ species observed in experiment. The heats of formation are presented for C4Fn and C4Fn+, n = 4-8.