A new type of viscometer based on the Poiseuille flow principle has been developed that is capable of measuring solution viscosities at ultrahigh pressures. The capillary time-of-flight (CTOF) viscometer has been used to measure the viscosity of methanol-water and acetonitrile-water mixtures in decade volume% increments from atmospheric pressure to 3500 bar (50,000 psi), at 25 degrees C. This instrument works by utilizing a relatively small pressure drop (approximately 200 bar) across a capillary which has both inlet and outlet pressurized so that the average column pressure can be significantly elevated (up to 3500 bar). Measurements from the CTOF viscometer match high-pressure viscosity data collected previously using falling-body viscometers of the Bridgman design. This manuscript serves to bring viscosity data at ultrahigh pressures for the two most common liquid chromatographic mobile phases into the chromatographic literature.