The parent allenediazonium monocation H₂CCCH(N₂⁺) and ten of its substituted derivatives XYCCC(Z)N₂⁺ (with F, CF₃, Me, OMe, and Me₂N as substituents) were studied by DFT at the B3LYP/6-31++G** level. Except for the Me₂N-substituted derivative that forms a monocation–N₂ complex, structurally intact allenediazonium ions were obtained as minima in all cases. Protonation studies at various sites were performed on allenediazonium cations, and relative energies of the resulting minima were used to identify the energetically most favored dications. In the majority of cases, protonation at the central carbon of the allenic moiety (C₂) is most favored, forming delocalized allyl cation–N₂⁺ species. The same dication structure is formed via initial C₃-protonation, followed by a formal hydride shift, in cases where a carbocation-stabilizing group is placed at C₃. When a CF₃ group is placed at C₃, initial protonation at C₁ resulted in a 1,3-fluorine shift, to generate a fluoroallyl cation linked to a CH₂N₂⁺ moiety. Structural features in the allenediazonium monocations and their protonated dications were examined, taking into account their geometrical features, computed charges, and the GIAO NMR shifts.