The viscosities of cyclohexane and decane are reported at temperatures between (303.15 and 598.15) K and at pressures of (0.1, 1, 2, 3 and 4) MPa. The experiments were carried out with a dual-capillary viscometer that measures the ratio of the viscosities at temperature T and pressure p to that at a reference temperature of 298.15 K and the same pressure. Absolute values of the viscosity were then obtained with an expanded relative uncertainties at 95 % confidence of 3.0 % by combining the measured ratios with literature values of the viscosity at the reference temperature.