An equation of state (EOS) that can represent solid-liquid, solid-vapor, and solid-liquid-vapor equilibria at planetary cryogenic conditions is presented. The EOS consists of two parts that are based on a common framework of the Thermodynamic Perturbation Theory: the fluid part that applies the Perturbed-Chain Statistical Associating Fluid Theory, and the solid part that applies the Lennard-Jones solid EOS. Our compelling results of the multiphase equilibria of chemical species and their mixtures encourage further development of the EOS to become a powerful tool in scientific investigations on objects in the outer-planet region. (C) 2013 Elsevier B.V. All rights reserved.