A new representation of the viscosity of ammonia is presented. The representative equations are based on a set of experimental data selected as a result of a critical assessment of the available information. The validity of the representation extends from 196 K to the critical temperature for both liquid and vapor phases. In the supercritical region the temperature range extends to 680 K for pressures at or below ambient and to 600 K for pressures up to 50 MPa. The accuracy of the representation varies from 0.5% for the viscosity of the dilute gas phase at moderate temperatures to about 5% for the viscosity at high pressures and temperatures. Tables of the viscosity generated by the correlating equation at selected temperatures and pressures and along the saturation line are presented to provide easy reference as well as for the validation of computer codes. ©1995 American Institute of Physics and American Chemical Society.