The viscosity of water containing dissolved CO2 at pressures up to 5 MPa was measured by a new method based on dynamic light scattering. The accuracy of this method was checked by measuring the temperature dependence of the viscosity of pure water at pressures up to 5 MPa and comparing the results to values in the literature that were determined by other methods. The discrepancy of the measured viscosity of water from the literature was +/-6%, which roughly equals the composite uncertainty of this method. The effect of dissolved CO2 on the viscosity was determined as functions of time, temperature, and pressure. The results indicated that the viscosity of the solution increased with increasing CO2 concentration. High viscosities were observed under conditions in which CO2 hydrates can form.