The OPLS force field for organic and biomolecular systems has been expanded to include an all-atom representation for saturated hydrocarbons. The model was formulated and tested via Monte Carlo simulations for both the pure Liquids and dilute aqueous solutions of methane, ethane, propane, and butane and as such leaves no ambiguity in terms of its performance in either hydrophilic or hydrophobic environments. To evaluate accuracy, key comparisons with available experimental thermodynamic and structural data including heats of vaporization, molecular volumes, fluctuational properties, and radial distribution functions for the liquids as well as free energies of hydration were made and found to be in reasonable accord. Revisitation of the united-atom model showed that the all-atom force field performs with overall superiority (e.g., average deviations from experimentally measured free energies of hydration were reduced from 0.36 to 0.13 kcal/mol). Comparisons of the results with those from other force fields are also provided.