Solutes can make supercritical solutions thicker, especially near critical pressure. Viscosity decreases as pressure rises due to solute-solute interactions. At high pressure, solvent-solvent interactions dominate, making viscosity similar to pure solvent. A modified equation predicts viscosity change based on solute volume fraction. A simple model calculates supercritical solution viscosity from solute size and pure fluid viscosity. Unexpectedly high viscosity at low pressures can affect reaction rates.