This dataset contains high-pressure, high-temperature (HPHT) rheological measurements of ionic liquids and alternative geothermal fluids. Experiments were conducted at Oklahoma State University using a Chandler Model 5550 viscometer and Rheo 5000 software to evaluate the flow behavior of deep eutectic solvents (DES), hexylpyridinium bromide ([HPy][Br]), water-based fluids, and oil-based fluids under varying thermal and pressure conditions. The rheological tests include both pure fluids and aqueous ionic liquid solutions at two concentrations (33.33% and 66.67% by weight in deionized water). Measurements were taken at constant pressures or temperatures, and data are organized into calibration records, raw viscosity measurements, and modeled rheological parameters. Tests were performed using nitrogen as the pressurization gas, with a sampling rate of 0.25 Hz.