Transport properties of viscosity of hydrogen-water vapor systems are needed for engineering applications in PEMFC system analysis and design. For instance, transport properties are needed up to 500 K for hydrogen fuel cell operation using PEMs. The properties may be conveniently represented as a function of the quantity of moisture by weight in grams of water per kilogram of dry hydrogen. The most recent values of viscosity of hydrogen based on a full quantum-mechanical formalism as well as the viscosity of water based on a new international formulation were used to calculate the effective viscosity of hydrogen-water mixture for constant pressures of 0.1 MPa, 0.5 MPa and 1 MPa, respectively, in the temperature range from 340 K to 500 K and in the range of water vapor concentrations from 100 grams of water vapor per kilogram of dry hydrogen to 100 kilograms of water vapor per kilogram of dry hydrogen. The results show that the effective viscosity of a hydrogen-water vapor mixture is not sensitive to the changes in ambient pressure in a PEMFC system. Their relative standard uncertainties are shown to be 0.2 % of the values with a coverage factor k = 2 (confidence level of 95 %).