Coiled two-capillary viscometers can measure gas viscosities with high accuracy over a wide temperature range. The authors built a two-capillary viscometer and used it to measure the zero-density viscosity of Kr and Xr at temperatures from 243.15 K to 393.15 K with relative standard uncertainties of 0.10% and 0.11%, respectively. Additional measurements on Ar were used to understand and avoid the error due to inadequate preheating or precooling of the gas entering the viscometer. The error, which occurred at volume flow rates above 4 m(3) s(-1), was approximately proportional to the temperature difference between the reference capillary and the probe capillary. (C) 2016 Elsevier B.V. All rights reserved.