Multireference Møller–Plesset perturbation theory with complete active space configuration interaction (CASCI-MRMP) is applied to the study of the valence π→π* excited states of all-trans linear polyenes C2nH2n+2 (n = 3–14). Our focus was to determine the nature of the four lowest-lying singlet excited states, 21Ag, 11Bu+, 11Bu and 31Ag and their ordering. The ionic 11Bu+ state is the lowest optically allowed excited state, while the covalent 21Ag, 11Bu and 31Ag states are the optically forbidden states. Theory predicts that the 11Bu state becomes lower than the 11Bu+ state at n ≥ 7 and that the 31Ag state also becomes lower than the 11Bu+ state at n ≥ 11.