Controlling the regioselectivity in the exohedral functionalization of fullerenes and endohedral metallofullerenes is essential to produce specific desired fullerene derivatives. In this work, using density functional theory (DFT) calculations, we show that the regioselectivity of the Diels–Alder (DA) cycloaddition of cyclopentadiene to ^2S+1C₆₀ changes from the usual [6,6] addition in the singlet ground state to the [5,6] attack in high spin states of C₆₀. Changes in the aromaticity of the five- and six-membered rings when going from singlet to high spin C₆₀ provide a rationale to understand this regioselectivity change. Experimentally, however, we find that the DA cycloaddition of isoindene to triplet C₆₀ yields the usual [6,6] adduct. Further DFT calculations and computational analysis give an explanation to this unanticipated experimental result by showing the presence of an intersystem crossing close to the formed triplet biradical intermediate.