The mixing of the two π-systems in quasi-D 2 symmetry mediates the optical activity. However, it appears that the rotatory strengths are not mediated by the MO-helicity as such. This is also evident by ⃗ µ and ⃗ m not pointing along the helical axis of the MOs, as they do for the allene and butatriene. Delocalization of the π-systems has been demonstrated to affect the optical properties of spiropentadienes, and this is also the case for the chiroptical response here. Although the optical activity is not driven by electrohelicity, we showed in a previous communication that the helicity manifests in the electron density, which changes in a helical fashion when charge is transferred from helical to nonhelical MOs during the four ππ * transitions.