The radical cation of quadricyclane (Q) was studied by pulse radiolysis at 133 K with methylcyclohexane (MCH) as solvent, saturated with N2O. The known solvent radical cations, MCH+ and its precursor M+*, are expected to produce Q(+) by charge transfer. Although Q(+) was known not to absorb in the visible lambda-range, there was a very early absorption band at lambda(max) = 720 nm (the transient is called Q(+)*), which eventually transformed into the cation of norbornadiene (NBD+) with lambda(max) = 650 nm. The analysis of the geminate ion kinetics with the semiempirical t(-0.6) kinetic law revealed that Q(+)* decays faster than the isomer NBD+ is built up. Q(+)* must be a precursor to the nonabsorbing Q(+), which eventually isomerizes to NBD+, followed by a back reaction with Q to re-form Q(+). The quantitative analysis revealed that a substantial amount of the cations is lost before NBD+ is formed. This loss to a fragment or isomer (called F+) occurs from Q(+)*. As this loss dropped drastically for very low [Q], Q(+)* must increasingly be bypassed by lowering [Q]. It turns out that Q(+)* is produced from M+* only (the higher energy precursor of the solvent radical cation MCH+) in competition with the transformation of M+* to MCH+, the latter becoming dominant at low [Q], increasingly producing Q(+) directly from MCH+ without going through Q(+)*. The loss yield (F+) correspondingly loses. The complete mechanism is given (Scheme 3). All the rate constants and the free ion contributions of all cations were determined and, together with the known G(fi) value, the absorption coefficients were derived. Comparing these results with a mechanism proposed recently by Adam et al. (J. Am. Chem. Sec. 1995, 117, 9693) suggests that Q(+)* corresponds to their cation Q(+)(l), where the lateral bonds are oxidized, Q(+) to their cation Q(+)(i), where the internal bonds are oxidized, and F+ the Q(+) isomer BHD+ (the bicyclo[3.2.0]hepta-2,6-diene cation). The precursor ion M+* of the solvent, which is responsible for the Q(+)* production must be of higher energy than MCH+; however, its structure remains unknown. The two precursor cations, M+* and Q(+)*, are critically compared and discussed.