Abstract Isotopic dating of geological samples using the K/Ar method and its variant, the 40 Ar/ 39 Ar dating technique, provides ages that in favourable circumstances are precise and accurate to within 1%. Limiting factors include the accuracy of the decay constants for 40 K and the age of neutron fluence monitor minerals. For rapidly cooled igneous rocks, a K/Ar or 40 Ar/ 39 Ar age normally will give a good estimate of the age of eruption, but for slowly cooled igneous rocks or metamorphic rocks an age measured on a sample is likely to be a cooling age. As 40 Ar*/ 39 Ar K ratios can now be determined often to better than 0.1%, the main limitation on accuracy relates to how well the 40 K decay constants are known. Better determination of the β − and γ decays of 40 K, the basis for the decay constants, is suggested at the present time, rather than adoption of new decay constants linked to another decay scheme. Until improved independently measured decay constants for 40 K become available, there may be some circumstances where the newly proposed decay constants need to be used, especially when comparing ages on volcanic igneous rocks with those measured in another system.