The viscosity of fourteen very pure high molecular weight hydrocarbon liquids has been determined at six temperatures, ranging from 37.78°C to 135°C, and at pressures ranging from atmospheric to 3400 bar, by use of a rolling-ball viscometer. For nine of the liquids whose PVT data were not available, a flexible bellows piezometer was used to determine the density between the same limits of temperature and pressure as employed for the viscosity measurements. The hydrocarbons selected vary in molecular weight from 268.5 to 535.0 and comprise several structural symmetry families involving progressive cyclization to cyclohexyl, phenyl, and decalyl groups. Also included are an alkylnaphthalene, an alkyldecahydronaphthalene, two hydrochrysenes, an homologous series of branched paraffins, and a series of 1,1-diphenylalkanes. Correlations are made of structure with viscosity, rate of change of viscosity with pressure, and enthalpy of activation for viscous flow. The effects of molecular weight and of molecular structure on viscosity are discussed.