Authigenic sanidine occurs in almost monomineralic beds in the basal St. Peter Sandstone near Waukau, Wisconsin. The high purity of this K-feldspar indicates that it formed at low temperature either contemporaneously with deposition or during later diagenesis. K-Ca analyses of two samples from the same bed yield discordant ages of 436 ±4 and 407 ±3 Ma. Because 40Ca loss is unlikely at the low temperatures to which these minerals have been subjected, these ages are interpreted as the times of formation of the sanidine. Both ages are younger than the estimated stratigraphic age of the St. Peter Sandstone. K-Ar analyses of splits of these same two samples give ages of 402 and 377 Ma, respectively. These ages are lower by about 8% than the K-Ca ages because of the diffusive loss of radiogenic argon over geologic time. An estimate of the diffusion coefficient for argon in the sanidine on the basis of the difference between the K-Ca and K-Ar ages suggests that diffusion of argon is about five orders of magnitude faster at low temperatures than an extrapolation of high-temperature experimental data would indicate. Comparison of K-Ca and K-Ar ages may be useful in general for distinguishing diffusive loss from recrystallization events in feldspars.