An equation which relates the volume term (V=M/(ρL-ρg)) of unassociated liquids to pressure P and temperature T has been obtained by the combination of (a) 03V(∂I/∂V)P[RIGHTWARDS ARROW]0=Tx-T for the effect of temperature on V at low (atmospheric) pressure and (b) - V(∂P/∂V)T = Px Vx/6/V6p[RIGHTWARDS ARROW]0+9(P-p) for the effect of pressure on volume at constant temperature. In the equations, p is the vapour pressure; pL the density of the liquid and pg the vapour density. Often pg can be neglected compared with pL and p is small compared with the large pressures required to affect the densities of liquids appreciably. There are three constants: Tx, Px, which equals 4.455 × 109 N m2, and Vx which can be calculated by the addition of atomic values for all the atoms in the molecule and subtraction of a value (6.56 × 106 m3 mol1) for each bond. When V approximates to ML, the molar volume, the equation can be integrated to give the work and heat of isothermal compression. The viscosity of a liquid is related to the work of compression and solubilities in a liquid to the work required to bring the solute to the compressibility of the liquid. Many relationships can be derived and can be used to estimate properties of unassociated liquids.