An equation of state for chain molecules is developed. It uses a hard-chain reference term based on the theory of Wertheim. Attractive interactions are described using a square-well potential with temperature-dependent well width. The proposed model accounts for the connectivity of the chain segments also in the attraction term using the analytical expression for the radial distribution function at contact for chain fluids derived by Chiew. For the description of a pure component three parameters are required : segment number, segment diameter and the square-well potential depth describing the segment-segment interactions. These parameters were estimated for numerous substances, Vapor pressures and Liquid densities of volatile fluids as well as polymer densities can be reproduced with high accuracy. A comparison with the original SAFT model and the modified GFD theory shows that the proposed equation of state can fit the pure-component behavior better than the other two models.