An equation-of-state (EOS) contribution for quadrupolar interactions is developed based on a third-order perturbation theory. Model constants are adjusted to comprehensive molecular simulation data for vapor-liquid equilibria of the two-center Lennard-Jones (2CLJ) plus pointquadrupole fluid from the literature. Molecular elongations L* from 0 (spherical) to 0.8 are covered by the simulation data. The EOS is suited for both, the 2CLJ fluid and the tangent-sphere Lennard-Jones framework. It is applied to pure components and mixtures of real substances with the perturbed-chain statistical associating fluid theory (PC-SAFT) EOS. It is possible to use tabulated values of quadrupolar moments (independently determined) directly in the EOS, so that no additional pure component parameter is introduced. Compared with the case where quadrupolar interactions are not specifically accounted for, the proposed model gives a systematic improvement of pure component properties and of mixture phase equilibria of strong quadrupolar components, such as carbon dioxide, and the absolute value of the required binary interaction parameter is significantly reduced. (c) 2005 American Institute of Chemical Engineers.