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