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<p><sup>40</sup>Ar/<sup>39</sup>Ar and K-Ar geochronology assume that <sup>40</sup>K/K values are invariant among the sample of interest, the co-irradiated neutron fluence monitor (<q>standard</q>), and the material used to measure decay constants. Until recently, this assumption was reasonable due to the small K isotope (<sup>41</sup>K, <sup>40</sup>K, <sup>39</sup>K) variability found in many terrestrial samples and the negligible effect of any variation relative to the precision of the determined age. The recent discovery of measurable δ<sup>41</sup>K variability in terrestrial samples now questions this assumption. Although δ<sup>41</sup>K values for some neutron fluence monitors have now been reported, potassium isotopes are not routinely measured on samples dated by the <sup>40</sup>Ar/<sup>39</sup>Ar method even though a wide range of silicate materials were found to vary by > 2.5 ‰. Further, the <sup>40</sup>K decay constants used in <sup>40</sup>Ar/<sup>39</sup>Ar geochronology are based on activity counting of radioactive decay in K-rich salts. These salts have not been measured for δ<sup>41</sup>K, yet evaporites have been shown to vary by > 1 ‰ from the mean value of silicates. The potential effects of δ<sup>41</sup>K variability on <sup>40</sup>Ar/<sup>39</sup>Ar ages are illustrated using the case of the ca. 28.2 Ma Fish Canyon sanidine (FCs) and the ca. 99 Ma Mt. Dromedary Biotite (GA-1550). If the two standards have δ<sup>41</sup>K values as measured and the material used to determine decay constants is appropriately represented by δ<sup>41</sup>K of evaporites, the age of FCs is underestimated by ca. 7 ka (0.25 ‰). Although this is a small effect, such bias is becoming important as the analytical precision and accuracy of isotopic measurements and calculation of <sup>40</sup>Ar/<sup>39</sup>Ar ages continue to improve.</p>