A non-classical structure for C4H7+ is supported by substantial experimental evidence from solvolysis experiments and NMR studies conducted in non-nucleophilic media. One or both of two structures, the cyclopropylcarbinyl cation and the bicyclobutonium cation, were invoked to account for the observed reactivity in various experiments, while the NMR data point to a highly fluxional system that undergoes rapid rearrangement to give an averaged spectrum consisting of only two 13C NMR signals, even at temperatures as low as –132 °C. Computationally, it was confirmed that the energetic landscape of the C4H7+ system is very flat, and that the two isomers postulated based on experimental data are very close in energy, the bicyclobutonium structure being computed to be just 0.4 kcal/mol more stable than the cyclopropylcarbinyl structure. In the solution phase (SbF5-SO2ClF-SO2F2, with SbF6– as the counterion), the bicyclobutonium structure predominates over the cyclopropylcarbinyl structure in a 84:16 ratio at –61 °C.