Measurements have been made, using a vibrating tube densitometer, of the effects of dissolved air, nitrogen, oxygen, hydrogen, methane, and carbon dioxide on the densities of liquid hydrocarbons at 298.15 K and a total pressure of 101060 Pa. The hydrocarbons were heptane, octane, nonane, decane, dodecane, tetradecane, hexadecane, cyclohexane, methylcyclohexane, and methylbenzene (toluene), Saturation of the liquids with gases other than carbon dioxide decreases the density by amounts ranging from about 0.004% for oxygen in hexadecane up to about 0.18 for methane in heptane. Saturation with carbon dioxide increases the densities by a maximum of almost 0.1%. For all gases, values of Delta rho decrease with increasing carbon number in an approximately linear manner in the alkane series. Partial molar volumes of the gaseous solutes have been calculated by combining the density results with literature gas solubility data.