Three open-chain isomers of C6H4+ and two cyclic ones were detected following mass-selective trapping in 6 K neon matrixes. The open-chain cations 5-hexene-1,3-diyne (CH2=CH-CC-CC-H)(+) and cis- (cis-HCC-CH=CH-CCH)(+) and trans-3-hexene-1,5-diyne (trans-HCC-CH=CH-CCH)(+), possess two absorption systems commencing at 609 and 373, 622 and 385, and 585 and 373 nm, respectively. They are assigned to the 1 (2)A '' and 2 (2)A '' <- (X) over tilde (2)A '', 1 (2)A(2) and 2 (2)A(2) <- (X) over tilde B-2(1) and 1 B-2(g) and 2 B-2(g) <- (X) over tilde (2)A(u) electronic transitions of these cations. Two overlapping systems are detected at around 420 nm and tentatively assigned to the 1 (2)A '' <- (X) over tilde (2)A '' electronic transitions of propargyl cyclopropene and 2 (2)A(1) <- (X) over tilde (2)A(2) of o-benzyne cation structures. The assignment of the electronic transitions is based on theoretical vertical excitation energies calculated with CASPT2 and EOMEE-CCSDT methods for 12 isomers of C6H4+. These have been carried out at the geometries optimized using several ab initio methods. Adiabatic excitation energies were calculated for the five identified isomers of C6H4+.