Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the valence π→π* excited states of 1,3-butadiene, 1,3,5-hexatriene, 1,3,5,7-octatetraene and 1,3,5,7,9-decapentaene. Our focus is put on determining the nature of the two lowest-lying singlet excited states, 11Bu+ and 21Ag, and their ordering. The 11Bu+ state is a singly excited state with an ionic nature originating from HOMO → LUMO one-electron transition while the covalent 21Ag state is the doubly excited state which comes mainly from (HOMO)2→(LUMO)2 transition. The active space and basis set effects are taken into account to estimate the excitation energies of larger polyenes. For butadiene the 1Bu+ state is calculated to be slightly lower by 0.1 eV than the doubly excited 21Ag state at the ground state equilibrium geometry. For hexatriene our calculations predict the two states to be virtually degenerate. Octatetraene is the first polyene for which we predict that the 21Ag state is the lowest excited singlet state at the ground state geometry. The present theory also indicates that the 21Ag state lies clearly below the 11Bu+ state in decapentaene with the energy gap of 0.4 eV. The 0–0 transition and the emission energies are also calculated using the planar C2h relaxed excited state geometries. The covalent 21Ag state is much more sensitive to the geometry variation than the ionic 11Bu+ state, which places the 21Ag state significantly below the 11Bu+ state at the relaxed geometry.