The applicability of chain equations of state to real systems is discussed in this paper. For this purpose, we have compared four theories : the perturbed-hard-chain theory (PHCT) of Prausnitz and co-workers, the generalized Flory (GF) and generalized Flory dimer (GFD) theories of Hall and co-workers, and the statistical associating fluid theory (SAFT) of Radosz, Gubbins, and co-workers. In our comparison, the perturbation expansion in the attractive term was truncated after the first-order term for all theories. Comparison of these theories with Monte Carlo simulation data for hard chains and square-well chains showed that the GFD theory, which explicitly takes into account the effect of the formation of chains in both the repulsive and the attractive part of the equation, is in best agreement with the data. The GFD theory was further improved by using the simulation data directly to reevaluate the shape parameters c and q, which were found to be density dependent. The new simplified GFD theory gives a significantly better correlation of the properties of normal alkanes than the other four theories.