A thermodynamic model based on the use of a modified free volume theory (MFVT) coupled with a cubic equation of state (CEoS) was developed here to obtain improved representations of the dynamic viscosity of some representative pure fluids (in both gaseous and dense states). In the original free volume theory, there is a barrier energy that is connected to the energy necessary for a molecule to cross a barrier and diffuse and is proportional to the density. In this work, we found that the aforementioned barrier energy was better expressed in terms of (rather than the density) a cohesive energy between molecules, namely the internal energy which accounts for all the intermolecular forces that hold the molecules together. The various thermodynamic potentials present in the resulting MFVT model (internal energy an density) were estimated from two simple cubic equations of state of the van der Waals type (Soave or Peng-Robinson). The present three-parameter model was successfully validated during the representation of experimental dynamic viscosities of two polar and associating fluids (water and methanol), one non-polar fluid (CO2) and one non-polar and long-chain fluid (n-decane) within temperature and pressure ranges covering both the compressed-liquid and dense-gas states.