Using two types of combined infrared/visible interferometers, the refractive index of moist air in the 3-μm region has been determined to an accuracy of 1.2 × 10^-8 by utilizing the value for visible light. The refractive index of water vapour also has been measured using one of the interferometers. Edlén's formula for dry air in the visible region is verified to be valid when extrapolated into the 3-μm region. On the other hand, the dispersion formula for visible light in water vapour needs some corrections when applied to the 3-μm region.