The gaseous viscosity of non-azeotropic mixtures of HCFC-22 and HCFC-142b was measured by an oscillating disk viscometer of the Maxwell type from 298.15 K to 423.15 K and at pressures up to 6.3 MPa. The viscosity at approximately atmospheric pressure was predicted with a maximum deviation of 1.84 % and an average deviation of 0.61 % by the Sutherland-Thiesen equation, coefficients of which were determined by the Brokaw method. An empirical equation was developed for the viscosity as a function of composition, temperature, and density. This equation reproduced the observed viscosity with a maximum deviation of 5.61 % and an average deviation of 1.14 %.