Publisher Summary This chapter discusses physical properties of fossil hydrocarbons. Active interest in the physical properties of light (C1–C5) hydrocarbons is, and remains, largely confined to their densities, specific heats, and related parameters, and to so-called “gas hydrates,” that is, clathrate compounds that form in certain circumstances by interaction with water. The almost limitless variety of crude oils that command industrial interest has, not surprisingly, focused attention on a substantially wider set of physical properties than are, in practice, required to be known for “natural gas” and this, in turn, has generated data that permit correlation with aspects of chemical composition. Such linkages, usually cast in the form of trend diagrams, characterize the oils, allow classification, and assist selection of optimal preparation and/or processing options. Parameters for trend diagrams are properties—mainly melting points, distillation ranges, densities, and viscosities—that vary systematically with average molecular weights (which reflect transitions from light to heavy oils). However, also often evaluated when characterizing crudes are certain “quality” indicators (color, odors, flash points, etc.) and specifics that bear on reservoir mechanics.