Valence-excited singlet (S-1,S-2) and triplet (T-1-T-4) states of acetylene have been studied by means of extended multireference electron correlation techniques (MR-CISD, MR-CISD+Q, and MR-AQCC). Extrapolations to the basis set limit have been performed. Minima and saddle points have been calculated using a recently developed analytic gradient method for excited states. Planar as well as nonplanar structures have been considered. In particular, the existence of an asymmetric, planar cis-type minimum on the S-2 surface has been confirmed conclusively. Moreover, an intersection S-1/S-2 has been located close to this minimum. This situation will most probably affect the interpretation of the absorption bands attributed to the trans 1 B-1(u) state. In-plane and out-of-plane saddle points for cis-trans isomerization have been determined and characterized by harmonic vibrational analysis. Several interesting surface crossings for different electronic states (S-1/S-2, T-2/T-3, and S-1/T-3) have been characterized. Implications of the flatness of the T-3 surface around linear structures and the location of the S-1/T-3 crossing seam on the anomalities observed in the ZAC spectrum of the (A) over tilde (1)A(u) state are discussed. (C) 2003 American Institute of Physics.