Equilibrium vapor pressures were determined by combining a head-space sampler with a gas chromatograph for the system cyclohexane/polystyrene in the temperature range from 35 to 65-degrees-C. From these measurements the Flory-Huggins interaction parameter chi, its variation with phi2, the volume fraction of the polymer, and its enthalpy part were calculated; chi(phi2) was represented by an analytical expression given by Koningsveld and Kleintjens and compared with different experimental findings reported in the literature. All measurements agree reasonably well at 35-degrees-C (in the vicinity of the THETA temperature of the system). Differences develop as T is raised; at 65-degrees-C chi and its concentration dependence are found to be less pronounced than reported. Under these conditions there also exists a discrepancy between the values calculated from chi(phi2) for vanishing polymer concentration and those directly measured. The reasons for that observation are discussed. The present work demonstrates that the new method yields quick and reliable information on chi(phi2;T).