To confirm that BCB radical cation 1 •+ is indeed responsible for reactivity, and to establish the mechanism of the [2π+2σ] cycloaddition reaction, we set about designing experiments that could provide a deeper understanding of the transformation described. Firstly, UV/vis spectroscopy of the individual reaction components revealed that the photocatalyst [Mes2Acr t Bu2]ClO4 is the only light absorbing species at λ = 425 nm, confirming that direct excitation of either BCB 1a or alkene 2a cannot be responsible for reactivity (Fig. ). Furthermore, Stern-Volmer quenching studies clearly demonstrated that BCB 1a is an effective quencher of the photocatalyst excited state, whereas alkene 2a gave no indication that it can interact with this excited state species (Fig. ). However, quenching was detected, albeit to a lesser extent than for 1a, upon the addition of styrene (4a). These observations are in full corroboration with the cyclic voltammetry (CV) experiments that were performed (Fig. ). Here, both BCB 1a (+1.79 V vs Ag/AgCl) and styrene 4a (+2.03 V vs Ag/AgCl) show oxidation peaks that were deemed accessible for the photocatalyst excited state, whereas 2a was observed to have an oxidation potential well outside this range (+2.36 V vs Ag/AgCl). Additionally, the quantum yield for the standard reaction was calculated to be φ = 3.7, revealing that this cycloaddition reaction can proceed via a radical chain mechanism. Overall, these results strongly suggest that, in the case of non-activated alkenes, BCB radical cation 1 •+ acts as the key intermediate in the [2π+2σ] cycloaddition reaction. Despite undergoing oxidation by the photocatalyst, radical cations arising from styrene-type alkenes were found, during DFT studies, to be unable to lead to product formation and so this alternative pathway could be eliminated as a possibility (see Supplementary Information for details).