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A significant sulfate artifact may be produced by the interaction of SO2(g) with basic particles on filters when the SO2 is not first removed by a denuder. Such an artifact could invalidate most historical sulfate measurements in regions with alkaline soil. To test this hypothesis, we compare measurements from Teflon filters without a denuder to measurements from nylon filters following a carbonate denuder from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. The data at 11 remote sites in the Desert Southwest show no evidence of a sulfate artifact. Using a normal error distribution, we show that any artifact is much less than 20 ng/m(3). In addition, data from 16 collocated samplers at Meadview, AZ, show that removing the SO2 by denuders does not change the measured sulfate. The mean difference between no-denuder and denuder is -4 +/- 10 ng/m(3) for Teflon filters and 3 +/- 17 ng/m(3) for nylon filters. Brigham Young University obtained large differences between their denuder and filter pack samplers at Meadview. Our analysis shows that these differences are explained better by a bias in their denuder sampler than by an SO2 artifact. Our analysis also shows that their measurements are too imprecise to determine an artifact less than about 500 ng/m(3).