Theta (Theta) conditions in binary and multicomponent polymer solutions are calculated using a perturbed hard-sphere-chain (PHSC) equation of state. Theta conditions are identified by roots of the spinodal criterion in the limit of infinite polymer molecular weight and zero polymer-segment fraction. Because the thermodynamic properties of mixtures are described by an equation of state, both upper and lower Theta temperatures can be calculated. For binary systems (one polymer and one solvent), experimentally determined Theta conditions and the critical solution temperature for a single polymer molecular weight are used to obtain equation-of-state parameters. Parameters obtained in this manner yield nearly quantitative representation of upper and lower Theta temperatures as a function of pressure and upper and lower critical solution temperatures as a function of polymer molecular weight. For ternary systems (two solvents and one polymer), the dependence of the Theta temperature on solvent composition can be described using parameters obtained from binary data.