Thirty-four artists' colorants brushed on watercolor or cellulose paper have been exposed for 12 weeks to 93 ± 5 parts per billion (ppb) of the air pollutant sulfur dioxide (SO₂) in purified air. These exposures were carried out in the dark at ambient temperature and humidity. Color changes (L*, a*, b*, and DELTASIGMA) were measured every week using a reflectance color analyzer. Color parameters (x, y, X, Y, Z, L*, a*, and b*) were also calculated from the 380-700 nm spectra, recorded with a reflectance spectrophotometer, of unexposed colorants and of colorants exposed to SO₂. Color changes measured by these two methods were in excellent agreement for both unexposed and SO₂-exposed colorants (near-unity slopes, correlation coefficients greater than0.9). Exposure to sulfur dioxide resulted in little or no color change except for one category of colorants, the triphenylmethanes basic fuchsin, brilliant green, and pararosaniline base. A second exposure to higher levels of SO₂ (920 ± 30ppb) for 2 weeks resulted in little additional color change, if any. A comparison is made of artists' colorant fading resulting from exposure to several air pollutants, including sulfur dioxide, ozone, nirtogen dioxide, nitric acid, formaldehyde, and peroxyacetyl nitrate. Implications for colorant-containing objects in museum collections are briefly discussed.