Abstract A crossover EOS and a classical EOS have been studied and compared. The crossover EOS was a modified Redlich-Kwong EOS with a crossover function implemented, which was taken from literature and modified with one additional parameter. The classical EOS was either a modified Redlich-Kwong cubic EOS (for non-associating fluids) or the same EOS with an association term taken from the chemical theory. The equations of state have been applied to water and to n -alkanes up to n -heptane in the temperature range of 300–900 K and pressure range of 0.1–40 MPa. The modified crossover EOS improves the description in the dense phase region, and gave the following root mean square deviations for the components from a fit to vapor pressure plus a critical volume shift: vapor pressure 0.23–1%, saturated liquid volume 2.4–3.9%, saturated gas volume 0.8–12% and PVT data in the single-phase region 3.8–6.1%. For the classical EOS the root means square deviations were vapor pressure 0.2–0.9%, saturated liquid volume 0.6–3.6%, saturated gas volume 2.7–11.2%, and PVT 10–30%. The modification of the crossover EOS brought an overall improvement compared to the crossover EOS from literature. One advantage of the crossover EOS as modified here is the ability to predict the volumetric properties of both two- and single-phase regions of higher n -alkanes from the vapor pressure fit.