Starting from the recently characterized radical cation of bicyclo[3.3.0]octa-2,6-diene-4,8-diyl, four additional dihydropentalene radical cations (DHP.+) can be formed by phototautomerization in Freon glasses and argon matrices where they can be characterized by optical (Freon, argon) and ESR spectroscopy (Freon), Two of these DHP isomers can be prepared independently, while the cations of the other two are identified by analogy of their spectra with those of related compounds. The electronic structure of 1,2-, 1,4-, and 1,5-DHP.+, which have linear and cross-conjugated triene pi-systems is discussed on the basis of their photoelectron and optical spectra and INDO/S calculations. The part of the C8H8.+ potential surface comprising all ten possible DHP.+ tautomers and some related valence isomers is explored by high-level ab initio calculations. An FMO-based set of rules for sigmatropic rearrangements in radical cations is presented and serves to rationalize the observed H-shifts.