Using the rolling-ball method the viscosity of seven pure hydrocarbons, having 25 or 26 carbon atoms, and three binary mixtures of them has been measured to 3450 bars at 37.8°, 60.0°, 98.8°, and 135°C. The compounds included isoparaffinic, cycloparaffinic, and aromatic types. The increase in viscosity with pressure was found to be strongly dependent on molecular structure. The viscosity-temperature coefficient 1/η(∂η/∂T)p increased with increased pressure while the viscosity-pressure coefficient 1/η(∂η/∂P)T decreased with increased temperature. The behavior of the binary mixtures corresponded within 5% over a range of 50–100 fold change in viscosity to that of the pure compounds equivalent to them in molecular weight and average molecular structure. This remarkable agreement is interpreted to mean that the viscosity of these compounds is some additive function of their constituent groups whether these groups are combined in the same or different molecules as long as the basic molecular symmetry is unchanged. The values of the Eyring theory ΔF±, ΔH±i, ΔS±, and ΔV± for these data are discussed. For the saturated compounds at constant temperature, an approximately linear relation was found between log η and [(v/v0)4—(v/v0)2] where v is the specific volume and η the absolute viscosity.